THE  N.  W.  HARRIS  LECTURES 
FOR  1914 


were  founded  in  1906  through  the  generosity  of 
Mr.  Norman  Wait  Harris  of  Chicago,  and  are  to 
be  given  annually.  The  purpose  of  the  lecture 
foundation  is,  as  expressed  by  the  donor,  "to 
stimulate  scientific  research  of  the  highest  type 
and  to  bring  the  results  of  such  research  before 
the  students  and  friends  of  Northwestern  Uni- 
versity, and  through  them  to  the  world.  By  the 
term  'scientific  research'  is  meant  scholarly  in- 
vestigation into  any  department  of  human  thought 
or  effort  without  limitation  to  research  in  the  so- 
called  natural  sciences,  but  with  a  desire  that 
such  investigation  should  be  extended  to  cover 
the  whole  field  of  human  knowledge." 


1907  Personalism.     Borden   P.   Bowne 

1908  University     Administration.       Charles     W. 
Eliot 

1910  The  Age  of  Mammals.     Henry  F.   Osborn 

1911  Democracy      and      Poetry.        Francis      B. 
Gummere 

1912  The  Milk  Question.     Milton  J.  Rosenau 

1913  The    Constitution    of    Matter.      Joseph    S. 
Ames 


NORMAN  W.  HARRIS  LECTURES  FOR  1914 
AT  NORTHWESTERN  UNIVERSITY 


HEREDITY  AND  ENVIRONMENT 

IN  THE 

DEVELOPMENT  OF  MEN 


BY 

EDWIN  GRANT  CONKLIN 

PROFESSOR  OF  BIOLOGY  IN 
PRINCETON  UNIVERSITY 


PRINCETON  UNIVERSITY  PRESS 
PRINCETON 

LONDON :  HUMPHREY  MILFORD 

OXFORD  UNIVERSITY  PRESS 

1915 


Copyright,  1915,  by 
PRINCETON  UNIVERSITY  PRESS 

Published  February,  1915 


PREFACE 

The  origin  of  species  was  probably  the  great- 
est biological  problem  of  the  past  century ;  the 
origin  of  individuals  is  the  greatest  biological 
subject  of  the  present  one.  The  many  incon- 
clusive attempts  to  determine  just  how  species 
arose  led  naturally  to  a  renewed  study  of  the 
processes  by  which  individuals  come  into 
existence,  for  it  seems  probable  that  the  prin- 
ciples and  causes  of  the  development  of  indi- 
viduals will  be  found  to  apply  also  to  the 
evolution  of  races.  As  the  doctrine  of  evolu- 
tion wrought  great  change  in  prevalent  be- 
liefs regarding  the  origin  and  past  history  of 
man,  so  present  studies  of  development  are 
changing  opinions  as  to  the  personality  of  man 
and  the  possibilities  of  improving  the  race. 
The  doctrine  of  evolution  was  largely  of  theo- 
retical significance,  the  phenomena  of  develop- 
ment are  of  the  greatest  practical  importance ; 
indeed  there  is  probably  no  other  subject  of 


PREFACE 

such  vast  importance  to  mankind  as  the  knowl- 
edge of  and  the  control  over  heredity  and 
development.  Within  recent  years  the  experi- 
mental study  of  heredity  and  development  has 
led  to  a  new  epoch  in  our  knowledge  of  these 
subjects,  and  it  does  not  seem  unreasonable  to 
suppose  that  in  time  it  will  produce  a  better 
breed  of  men. 

The  lectures  which  compose  this  volume  were 
given  at  Northwestern  University  in  February, 
1914,  on  the  Norman  W.  Harris  Foundation 
and  were  afterward  repeated  at  Princeton  Uni- 
versity. I  gladly  take  this  opportunity  of  ex- 
pressing to  the  faculties,  students  and  friends 
of  both  institutions  my  deep  appreciation  of 
their  interest  and  courtesy.  In  attempting  to 
present  to  a  general  audience  the  results  of  re- 
cent studies  on  heredity  and  development,  with 
special  reference  to  their  application  to  man, 
the  author  has  had  to  choose  between  simplicity 
and  sufficiency  of  statement,  between  apparent 
dogmatism  and  scientific  caution,  between  a 
popular  and  a  scientific  presentation.  These 
are  hard  alternatives,  but  the  first  duty  of  a 
lecturer  is  to  address  his  audience  and  to  make 


PREFACE 

his  subject  plain  and  interesting,  if  he  can, 
rather  than  to  talk  to  the  scientific  gallery  over 
the  heads  of  the  audience.  In  preparing  the 
lectures  for  publication  it  has  not  been  possible 
to  avoid  the  technical  treatment  of  certain  sub- 
jects, but  in  the  main  the  lectures  are  still 
addressed  to  the  audience  rather  than  to  the 
scientific  gallery.  Unfortunately  biology  is 
still  a  strange  subject  to  many  intelligent 
people  and  its  terminology  is  rather  terrifying 
to  the  uninitiated ;  but  it  is  hoped  that  the  glos- 
sary at  the  end  of  the  volume  may  rob  these 
unfamiliar  terms  of  many  of  their  terrors. 

I  take  this  opportunity  of  thanking  Dr.  W. 
E.  Castle  and  Dr.  J.  H.  McGregor  for  the 
use  of  photographs  which  are  reproduced  in 
Figures  81,  82  and  99;  and  I  wish  especially  to 
thank  my  assistant,  Marguerite  Ruddiman, 
for  her  aid  in  preparing  figures  and  manu- 
script for  publication. 

Princeton,  December,  1914 


vii 


CONTENTS 

CHAPTER  I.    FACTS  AND  FACTORS  OF  DEVEL- 
OPMENT 

INTRODUCTION 

A.  PHENOMENA  OF  DEVELOPMENT 

I.  DEVELOPMENT  OF  THE  BODY 

1.  The  Germ  Cells 

2.  Fertilization 

3.  Cleavage 

4.  Embryogeny 

5.  Organogeny 

6.  Oviparity  and  Viviparity 

7.  Development  of  Functions 

II.  DEVELOPMENT  OF  THE  MIND 

1 .  Sensitivity 

2.  Tropisms,   Reflexes,   Instincts 

3.  Memory 

4.  Intellect,   Reason 

5.  Will 

6.  Consciousness 

B.  FACTORS  OF  DEVELOPMENT 

1.  Preformation 

2.  Epigenesis 

3.  Preformation  and  Epigenesis 

4.  Heredity  and  Environment 

ix 


CONTENTS 

Other  Mendelian  Ratios 

2.  Results    of    Crossing    Individuals    with 
more    than    one    pair     of     contrasting 
characters 

Dihybrids  and  Trihybrids 

3.  Inheritance  Formulae 

4.  Presence  and  Absence  Hypothesis 

5.  Summary  of  Mendelian  Principles 

a.  The  Principle  of  Unit  Characters 

b.  The  Principle  of  Dominance 

c.  The  Principle  of  Segregation 

II.  MODIFICATIONS   AND   EXTENSIONS   OF   MEN- 
DEHAN   PRINCIPLES 

1.  The  Principle  of  Unit  Characters  and 
Inheritance  Factors 

Inheritance      Factors      and      Germinal 
Units 

2.  Modifications  of  the  Principle  of  Domi- 
nance 

Sex  and  Sex  Limited  Inheritance 
Sex  Linked  Inheritance 

3.  The  Principle  of  Segregation 
"Blending"   Inheritance 

III.  Mendelian  Inheritance  in  Man 

CHAPTER  IV.    INFLUENCE  OF  ENVIRONMENT 

A.  RELATIVE  IMPORTANCE  OF  HEREDITY 
AND  ENVIRONMENT 

1.  Former  Emphasis  on  Environment 

2.  Present  Emphasis   on  Heredity 

3.  Both  Indispensable  to  Development 

xii 


CONTENTS 

B.  EXPERIMENTAL    MODIFICATIONS    OF 
DEVELOPMENT 

I.  DEVELOPMENTAL    STIMULI 

1.  Physical  Stimuli 

2.  Chemical  Stimuli 

II.  DEVELOPMENTAL  RESPONSES 

Dependent  upon  (a)  Nature  of  Organism, 
(b)  Nature  of  Stimulus,  (c)  Stage  of  De- 
velopment 

1.  Modifications     of    Germ    Cells    before 
Fertilization 

2.  During  Fertilization 

3.  After  Fertilization 

C.  FUNCTIONAL  ACTIVITY  AS  A  FACTOR 
OF  DEVELOPMENT 

D.  INHERITANCE  OR  NON-INHERITANCE 
OF  ACQUIRED  CHARACTERS 

E.  APPLICATIONS  TO  HUMAN  DEVELOP- 
MENT:    EUTHENICS 

CHAPTER  V.     CONTROL  OF  HEREDITY:     EU- 
GENICS 

A.  DOMESTICATED  ANIMALS  AND  CULTI- 
VATED PLANTS 
I.  INFLUENCE  OF  ENVIRONMENT  IN  PRODUCING 

NKW  RACES 
II.  ARTIFICIAL  SELECTION 

1.  The  Methods  of  Breeders 

2.  How  has  Selection  acted? 
III.  METHODS  OF  MODERN  GENETICS 

1.  Mendelian    Association    and    Dissocia- 
tion of  Characters 

2.  Origin  of  Mutations 

xiii 


CONTENTS 

B.  CONTROL  OF  HUMAN  HEREDITY 

I.  PAST  EVOLUTION  OF  MAN 
II.  CAN  HUMAN  EVOLUTION  BE  CONTROLLED? 

1.  Selective  Breeding  the  only  Method  of 
Improving  the  Race. 

2.  No   Improvement  in   Human   Heredity 
within  Historic  Times 

3.  Why  the  Race  has  not  Improved 
III.  EUGENICS 

1.  Possible  and  Impossible  Ideals 

2.  Negative  Eugenical  Measures 

3.  Positive  Eugenical  Measures 

4.  Contributory  Eugenical  Measures 

5.  The  Declining  Birthrate 
CHAPTER  VI.    GENETICS  AND  ETHICS 

I.  THE    VOLUNTARISTIC    CONCEPTION    OF    NA- 
TURE AND  OF  HUMAN  RESPONSIBILITY 
II.  THE  MECHANISTIC  CONCEPTION  OF  NATURE 
AND  OF  PERSONALITY 

1.  The  Determinism  of  Heredity 

2.  The  Determinism  of  Environment 
III.   DETERMINISM  AND  RESPONSIBILITY 

1.  Determinism  not  Fatalism 

2.  Control  of  Phenomena  and  of  Self 

3.  Birth  and  Growth  of  Freedom 

4.  Responsibility  and  Will 

5.  Our  Unused  Talents 

IV.  THE  INDIVIDUAL  AND  THE  RACE 

1.  The  Conflict  between  the  Freedom  of 
the  Individual  and  the  Good  of  Society 

2.  Perpetuation  and   Improvement  of  the 
Race  the  Highest  Ethical  Obligation 

REFERENCES,  GLOSSARY,  INDEX 
xiv 


CHAPTER  I 

FACTS  AND  FACTORS  OF 
DEVELOPMENT 


CHAPTER  I 

FACTS   AND   FACTORS   OF   DEVELOPMENT 

INTRODUCTION 

One  of  the  greatest  results  of  the  doctrine  of 
organic  evolution  has  been  the  determination  of 
man's  place  in  nature.  For  many  centuries  it 
has  been  known  that  in  bodily  structure  man  is 
an  animal;  that  he  is  born,  nourished  and  de- 
veloped, that  he  matures,  reproduces  and  dies 
just  as  does  the  humblest  animal  or  plant.  For 
centuries  it  has  been  known  that  man  belongs 
to  that  group  of  animals  which  have  backbones, 
the  vertebrates;  to  that  class  which  have  hair 
and  suckle  their  young,  the  mammals,  and  to 
that  order  which  have  grasping  hands,  flat 
nails,  and  thoracic  mammae,  the  primates,  a 
group  which  includes  also  the  monkeys  and 
apes.  But  as  long  as  it  was  supposed  that 
every  species  was  distinct  in  its  origin  from 
every  other  one,  and  that  each  arose  by  a 

3 


4  HEREDITY  AND  ENVIRONMENT 

special  divine  fiat,  it  was  possible  to  maintain 
that  man  was  absolutely  distinct  from  the  rest 
of  the  animal  world  and  that  he  had  no  kinship 
to  the  beasts,  though  undoubtedly  he  was  made 
in  their  bodily  image.  But  with  the  establish- 
ment of  the  doctrine  of  organic  evolution  this 
resemblance  between  man  and  the  lower  ani- 
mals has  come  to  have  a  new  significance.  The 
almost  universal  acceptance  of  this  doctrine  by 
scientific  men,  the  many  undoubted  resem- 
blances between  man  and  the  lower  animals, 
and  the  discovery  of  the  remains  of  lower  types 
of  man,  real  "missing  links,"  have  inevitably  led 
to  the  conclusion  that  man  also  is.  a  product  of 
evolution,  that  he  is  a  part  of  the  great  world  of 
living  things  and  not  a  being  who  stands  apart 
in  solitary  grandeur  in  some  isolated  sphere. 

But  wholly  aside  from  the  doctrine  of  evolu- 
tion, the  fact  that  essential  and  fundamental 
resemblances  exist  among  all  kinds  of  organ- 
isms can  not  fail  to  impress  thoughtful  men. 
Life  processes  are  everywhere  the  same  in 
principle,  though  varying  greatly  in  detail. 
All  the  general  laws  of  life  which  apply  to 
animals  and  plants  apply  also  to  man.  This 


FACTS  AND  FACTORS  OF  DEVELOPMENT  5 

is  no  mere  logical  inference  from  the  doc- 
trine of  evolution,  but  a  fact  which  has  been 
established  by  countless  observations  and  ex- 
periments. The  essential  oneness  of  all  life 
gives  a  direct  human  interest  to  all  living 
things.  If  "the  proper  study  of  mankind  is 
man,"  the  proper  study  of  man  is  the  lower 
organisms  in  which  life  processes  are  reduced 
to  their  simplest  terms,  and  where  alone  they 
may  be  subjected  to  conditions  of  rigid  ex- 
perimentation. Upon  this  fundamental  like- 
ness in  the  life  processes  of  man  and  other 
animals  are  based  the  wonderful  advances  in 
experimental  medicine,  which  may  be  counted 
among  the  greatest  of  all  the  achievements  of 
science. 

The  experimental  study  of  heredity,  develop- 
ment and  evolution  in  forms  of  life  below  man 
must  certainly  increase  our  knowledge  of  and 
our  control  over  these  processes  in  the  human 
race.  If  human  heredity,  development  and 
evolution  may  be  controlled  to  even  a  slight 
extent,  we  may  expect  that  sooner  or  later  the 
human  race  will  be  changed  for  the  better.  At 
least  no  other  scheme  of  social  betterment  and 


6  HEREDITY  AND  ENVIRONMENT 

race  improvement  can  compare  for  thorough- 
ness, permanence  of  effect,  and  certainty  of 
results,  with  that  which  attempts  to  change 
the  natures  of  men  and  to  establish  in  the  blood 
the  qualities  which  are  desired.  We  hear 
much  nowadays  about  man's  control  over  na- 
ture, though  in  no  single  instance  has  man 
ever  changed  any  law  or  principle  of  nature. 
What  man  can  do  is  to  put  himself  into  such 
relations  to  natural  phenomena  that  he  may 
profit  by  them,  and  all  that  can  be  done  to- 
ward the  improvement  of  the  human  race  is  to 
apply  consciously  to  man  those  great  princi- 
ples of  development  and  evolution  which  have 
been  operating  unknown  to  man  through  all 
the  ages. 

A.    PHENOMENA  OF  DEVELOPMENT 

One  of  the  greatest  and  most  far-reaching 
themes  which  have  ever  occupied  the  minds  of 
men  is  the  problem  of  development.  Whether 
it  be  the  development  of  an  animal  from  an 
egg,  of  a  race  or  species  from  a  pre-existing 
one,  or  of  the  body,  mind  and  institutions  of 
man,  this  problem  is  everywhere  much  the 


FACTS  AND  FACTORS  OF  DEVELOPMENT          7 

same  in  fundamental  principles,  and  knowl- 
edge gained  in  one  of  these  fields  must  be  of 
value  in  each  of  the  others.  Ontogeny  and 
phylogeny  are  not  wholly  distinct  phenomena, 
but  are  only  two  aspects  of  the  one  general 
process  of  organic  development.  The  evolu- 
tion of  races  and  of  species  is  sufficiently  rare 
and  unfamiliar  to  attract  much  attention  and 
serious  thought;  while  the  development  of  an 
individual  is  a  phenomenon  of  such  universal 
occurrence  that  it  is  taken  as  a  matter  of  course 
by  most  people,  something  so  evident  that  it 
seems  to  require  no  explanation;  but  famili- 
arity with  the  fact  of  development  does  not 
remove  the  mystery  which  lies  back  of  it, 
though  it  may  make  plain  many  of  the  proc- 
esses concerned.  The  development  of  a  human 
being,  of  a  personality,  from  a  germ  cell  is  the 
climax  of  all  wonders,  greater  even  than  that 
involved  in  the  evolution  of  a  species  or  in  the 
making  of  a  world. 

The  fact  of  development  is  everywhere  ap- 
parent ;  its  principal  steps  or  stages  are  known 
for  thousands  of  animals  and  plants ;  even  the 
precise  manner  of  development  and  its  factors 


8  HEREDITY  AND  ENVIRONMENT 

or  causes  are  being  successfully  explored.  Let 
us  briefly  review  some  of  the  principal  events 
in  the  development  of  animals,  and  particu- 
larly of  man,  and  then  consider  some  of  the 
chief  factors  and  processes  of  development. 
Most  of  our  knowledge  in  this  field  is  based 
upon  a  study  of  the  development  of  animals 
below  man,  but  enough  is  now  known  of  hu- 
man development  to  show  that  in  all  essential 
respects  it  resembles  that  of  other  animals, 
and  that  the  problems  of  heredity  and  differ- 
entiation are  fundamentally  the  same  in  man 
as  in  other  animals. 

I.  DEVELOPMENT  OF  THE  BODY 

The  entire  individual  structure  and  func- 
tions, body  and  mind  develops  as  a  single  in- 
divisible unity,  but  for  the  sake  of  clarity  it  is 
desirable  to  deal  with  one  aspect  of  the  indi- 
vidual at  a  time.  For  this  reason  we  shall 
consider  first  the  development  of  the  body,  and 
then  the  development  of  the  mind. 

1.  The  Germ  Cells. — In  practically  all  ani- 
mals and  plants  individual  development  begins 
with  the  fertilization  of  a  female  sex  cell,  or 


FACTS  AND  FACTORS  OF  DEVELOPMENT          9 

egg,  by  a  male  sex  cell,  or  spermatozoon.  The 
epigram  of  Harvey,  "Omne  vivum  ex  ovo>' 
has  found  abundant  confirmation  in  all  later 
studies.  Both  egg  and  spermatozoon  are  alive 
and  manifest  all  the  general  properties  of  liv- 
ing things.  How  little  this  fact  is  appreciated 
by  the  public  is  shown  by  the  repeated  an- 
nouncements by  the  newspapers  that  "Profes- 
sor So-and-so  has  created  life  because  he  has 
made  an  egg  develop  without  fertilization." 
An  egg  or  a  spermatozoon  is  as  much  alive  as 
is  any  other  cell;  as  characteristically  alive  as 
is  the  adult  animal  into  which  it  develops. 

It  is  difficult  to  define  life,  as  it  is  also  to  de- 
fine matter,  energy,  electricity,  or  any  other 
fundamental  phenomenon,  but  it  is  possible  to 
describe  in  general  terms  what  living  things 
are  and  what  they  do.  Every  living  thing 
whatever,  from  the  smallest  and  simplest 
micro-organism  to  the  largest  and  most  com- 
plex animal,  from  the  microscopic  egg  or 
spermatozoon  to  the  adult  man,  manifests  the 
following  distinctive  properties: 

(a)  It  contains  protoplasm,  "the  material 
basis  of  life,"  which  is  composed  of  the  most 


10  HEREDITY  AND  ENVIRONMENT 

complex  substances  known  to  chemistry. 
Protoplasm  is  not  a  homogeneous  substance, 
but  it  always  exists  in  the  form  of  cells,  which 
are  minute  masses  of  protoplasm  composed  of 
many  distinct  parts,  the  most  important  of 
these  being  the  nucleus  and  the  cytoplasm 
( Fig.  1 ) .  Protoplasm  is  therefore  organized, 
that  is,  composed  of  many  parts  all  of  which 
are  integrated  into  a  single  system,  the  cell. 
Higher  animals  and  plants  are  composed  of 
multitudes  of  cells,  differing  more  or  less  from 
one  another,  which  are  bound  together  and 
integrated  into  a  single  organism.  Living  cells 
and  organisms  are  not  static  structures  which 
are  fixed  and  stable  in  character,  but  they  are 
systems  which  are  undergoing  continual 
change.  They  are  like  the  river,  or  the  whirl- 
pool, or  the  flame,  which  are  never  at  two  con- 
secutive moments  composed  of  the  same 
particles  but  which  nevertheless  maintain  a 
constant  general  appearance ;  in  short  they  are 
complex  systems  in  dynamic  equilibrium. 

The  principal  physiological  processes  by 
which  all  living  things  maintain  this  equilib- 
rium are: 


FACTS  AND  FACTORS  OF  DEVELOPMENT    11 


N. 


Memb 


r 


FIG.  1.  A  nearly  ripe  human  ovum  in  the  living  condition. 
The  ovum  is  surrounded  by  a  series  of  follicle  cells  (FC)  in- 
side of  which  is  the  clear  membrane  (Hemb.)  and  within  this 
is  the  ovum  proper  containing  yolk  granules  (Y)  and  a  nucleus 
(N)  embedded  in  a  clear  mass  of  protoplasm.  Magnified  500 
diameters  (x  500).  (From  O.  Hertwig.)  B,  two  human 
spermatozoa  drawn  to  about  the  same  scale  of  magnification. 
(After  G.  Retzius.) 


12  HEREDITY  AND  ENVIRONMENT 

(b)  Metabolism,  or  the  transformation  of 
matter  and  energy  within  the  living  thing,  in 
the  course  of  which  some  substances  are  oxi- 
dized into  waste  products,  with  the  liberation 
of  energy,  while  other  substances  are  built  up 
into  protoplasm,   each  part  of  the  cell  con- 
verting food  substances  into  its  own  particular 
substance  by  the  process  of  assimilation. 

(c)  Reproduction,  or  the  capacity  of  organ- 
isms to  give  rise  to  new  organisms,  of  cells  to 
give  rise  to  other  cells,  and  of  parts  of  cells  to 
give  rise  to  similar  parts  by  the  process  of 
division. 

(d)  Irritability,  or  the  capacity  of  receiving 
and    responding    to    impinging    energies,    or 
stimuli,  in  a  manner  which  is  usually,  but  not 
invariably,  adaptive  or  useful. 

Both  the  egg  and  the  sperm  are  living  cells 
with  typical  cell  structures  and  functions,  but 
with  none  of  the  parts  of  the  mature  organism 
into  which  they  may  develop.  But  although 
they  do  not  contain  any  of  the  differentiated 
structures  and  functions  of  the  developed  or- 
ganism, they  differ  from  other  cells  in  that 
they  are  capable  under  suitable  conditions  of 


FACTS  AND  FACTORS  OF  DEVELOPMENT         13 

producing  these  structures  and  functions  by 
the  process  of  development  or  differentiation, 
in  the  course  of  which  the  general  structures 
and  functions  of  the  germ  cells  are  converted 
into  the  specific  structures  and  functions  of  the 
mature  animal  or  plant. 

In  both  plants  and  animals  the  sex  cells 
are  fundamentally  alike,  though  they  differ 
greatly  in  appearance.  The  female  sex  cells 
of  flowering  plants  are  called  ovules,  the  male 
cells  pollen.  The  corresponding  cells  of  ani- 
mals are  known  as  ova  and  spermatozoa.  Col- 
lectively all  kinds  of  sex  cells  are  called 
gametes,  and  the  individual  formed  by  the 
union  of  a  male  and  female  gamete  is  known 
as  a  zygote,  while  the  cell  formed  by  the  union 
of  egg  and  sperm  is  frequently  called  the 
oosperm. 

The  egg  cell  of  animals  is  usually  spherical 
in  form  and  contains  more  or  less  food  sub- 
stance in  the  form  of  yolk ;  it  varies  greatly  in 
size,  depending  chiefly  upon  the  quantity  of 
yolk,  from  the  great  egg  of  a  bird,  in  which 
the  yolk  or  egg  proper  may  be  hundreds  of 
millimeters  in  diameter,  to  the  miscroscopic 


14  HEREDITY  AND  ENVIRONMENT 

eggs  of  oysters  and  worms,  which  may  be  no 
more  than  a  few  thousandths  of  a  millimeter 
in  diameter.  The  human  ovum  (Fig.  1)  is 
microscopic  in  size  (about  0.2  mm.  in  diameter) 
but  it  is  not  smaller  than  is  found  in  many 
other  animals.  It  has  all  the  characteristic 
parts  of  any  egg  cell,  and  can  not  be  distin- 
guished microscopically  from  the  eggs  of  sev- 
eral other  mammals,  yet  there  is  no  doubt  that 
the  ova  of  each  species  differ  from  those  of 
every  other  species,  and  later  we  shall  see 
reasons  for  concluding  that  the  ova  produced 
by  each  individual  are  different  from  those 
produced  by  any  other  individual. 

The  sperm,  or  male  gamete,  is  among  the 
smallest  of  all  cells  and  is  usually  many  thou- 


...    T  ...  M     H 

A  --'"""  "****•-  ''' 

A  — _____ a* 


B 

FIG.  2.  Two  HUMAN  SPERMATOZOA.  A,  showing  the  surface 
of  the  flattened  head;  B,  its  edge;  H,  head;  M,  middle-piece; 
T,  tail.  (After  G.  Retzius.) 


FACTS  AND  FACTORS  OF  DEVELOPMENT         15 

sands  of  times  smaller  than  the  egg.  In  most 
animals,  and  in  all  vertebrates,  it  is  an 
elongated,  thread-like  cell  with  an  enlarged 
head  which  contains  the  nucleus,  a  smaller 
middle-piece,  and  a  very  long  and  slender  tail 
or  flagellum,  by  the  lashing  of  which  the  sper- 
matozoon swims  forward  in  the  jerking  fashion 
characteristic  of  many  monads  or  flagellated 
protozoa.  In  different  species  of  animals  the 
spermatozoa  often  differ  in  size  and  appear- 
ance, and  there  is  every  reason  to  believe  that 
the  spermatozoa  of  each  species  are  peculiar  in 
certain  respects  even  though  we  may  not  be 
able  to  distinguish  any  structural  differences 
under  the  microscope.  The  human  sperma- 
tozoa (Fig.  2)  closely  resemble  those  of  other 
primates  but  are  still  slightly  different,  and  the 
conclusion  is  logically  inevitable,  as  we  shall 
see  later,  that  the  spermatozoa  as  well  as  the 
ova  of  each  individual  differ  slightly  from 
those  of  every  other  individual. 

2.  Fertilization. — If  a  spermatozoon  in  its 
swimming  comes  into  contact  with  a  ripe  but 
unfertilized  egg,  the  head  and  middle-piece  of 
the  sperm  sink  into  the  egg  while  the  tail  is 


16  HEREDITY  AND  ENVIRONMENT 

usually  broken  off  and  left  outside.  The 
nucleus  in  the  head  of  the  sperm  then  begins 
to  absorb  material  from  the  egg  and  to  grow 
in  size  and  at  the  same  time  a  minute  granule, 
the  centrosome,  appears,  either  from  the  mid- 
dle-piece or  from  the  head  of  the  sperm,  and 
radiating  lines  run  out  from  the  centrosome 
into  the  substance  of  the  egg.  The  sperm 
nucleus  and  centrosome  then  approach  the  egg 
nucleus  and  ultimately  the  two  nuclei  come  to 
lie  side  by  side  (Figs.  4-7) .  Usually  when  one 
spermatozoon  has  entered  an  egg  all  others  are 
barred  from  entering,  probably  by  some 
change  in  the  chemical  substances  given  out  by 
the  egg. 

This  union  of  a  single  spermatozoon  with  an 
egg  is  known  as  fertilization.  Whereas  egg 
cells  are  usually,  but  not  invariably,  incapable 
of  development  without  fertilization,  there  be- 
gins, immediately  after  fertilization,  a  long 
series  of  transformations  and  differentiations 
of  the  fertilized  egg  which  leads  to  the  develop- 
ment of  a  complex  animal — of  a  person.  In 
the  fusion  of  the  egg  and  sperm  a  new  indi- 
vidual, the  oosperm,  comes  into  being.  The 


5     . 


FIGS.  4-5.  Two  STAGES  IN  THE  ENTRANCE 
OF  THE  SPERMATOZOON  INTO  THE  EGG  OF  hereis. 
Some  of  the  protoplasm  of  the  egg  has  gath- 
ered at  the  point  of  entrance  to  form  the 
entrance  cone  (EC)  which,  together  with  the 
sperm  head,  moves  into  the  interior  of  the 
egg  in  later  stages.  The  black  spheres  rep- 
resent yolk.  (From  F.  R.  Lillie.) 
FIG.  3.  ENTIRE  SPERMATOZOON  OF  THE  ANNELID  Nereis, 

showing  perforatorium  (P) ;  head  (H)  ;  middle-piece  (M),  and 

tail   (T).     (From  F.  R.  Lillie.) 


18  HEREDITY  AND  ENVIRONMENT 

oosperm,  formed  by  the  union  of  the  two  sex 
cells,  is  really  a  double  cell,  since  parts  of  the 
egg  and  sperm  never  lose  their  identity,  and 
the  individual  which  develops  from  this 
oosperm  is  a  double  being;  even  in  the  adult 
man  this  double  nature,  caused  by  the  union  of 
egg  and  sperm,  is  never  lost. 

In  by  far  the  larger  number  of  animal 
species  the  oosperm,  either  just  before  or 
shortly  after  fertilization,  is  set  free  to  begin 
its  own  individual  existence,  and  in  such  cases 
it  is  perfectly  clear  that  the  fertilization  of  the 
egg  marks  the  beginning  of  the  new  individual. 
But  in  practically  every  class  of  animals  there 
are  some  species  in  which  the  fertilized  egg  is 
retained  within  the  body  of  the  mother  for  a 
varying  period  during  which  development  is 
proceeding.  In  such  cases  it  is  not  quite  so 
evident  that  the  new  individual  comes  into  be- 
ing with  the  fertilization  of  the  egg;  rather 
the  moment  of  birth  or  the  separation  from 
the  mother  is  generally  looked  upon  as  the  be- 
ginning of  the  individual  existence.  And  yet 
in  all  cases  the  egg  or  embryo  is  always  dis- 
tinguishable from  the  body  of  the  mother  and 


FACTS  AND  FACTORS  OF  DEVELOPMENT         19 

there  is  no  protoplasmic  connection  between 
'the  two.  In  mammals  generally,  including 
also  the  human  species,  not  a  strand  of  proto- 
plasm, not  a  nerve  fiber,  not  a  blood  vessel 
passes  over  from  the  mother  to  the  embryo; 
the  latter  is  from  the  moment  of  fertilization 
of  the  egg  a  distinct  individual  with  particular 
individual  characteristics,  and  this  is  just  as 
true  of  viviparous  animals  in  which  the  egg 
undergoes  a  part  of  its  development  within 
the  body  of  the  mother  as  it  is  of  oviparous 
forms  in  which  the  eggs  are  laid  before  devel- 
opment begins. 

The  fertilized  egg  of  a  star-fish,  or  frog,  or 
man  is  not  a  different  individual  from  the 
adults  of  these  forms,  rather  it  is  a  star-fish,  a 
frog,  or  a  human  being  in  the  one-celled  stage, 
and  thereafter  this  new  being  maintains  its 
own  individuality.  This  fertilized  egg  fuses 
with  no  other  cells,  it  takes  into  itself  no  living 
substance,  but  manufactures  its  own  proto- 
plasm from  food  substances;  it  receives  food 
and  oxygen  from  without  and  it  gives  out 
carbonic  acid  and  other  waste  products;  it  is 
sensitive  to  certain  alterations  in  the  environ- 


l«t  Jfat. 


^*=£§^'u-  • .    *?^&&^ 


P'or  description  see  opposite  page. 


1st  PB 


9 

FIGS.  6-9.  SUCCESSIVE  STAGES  IN  THE  MATURATION  AND  FERTI- 
LIZATION OF  THE  EGG  OF  Nereis,  less  highly  magnified  than 
Figs.  6  and  7,  showing  the  progress  of  the  entrance  cone  (EC) 
and  sperm  nucleus  (£N)  into  the  egg.  Fig.  6  shows  the  first 
maturation  spindle  of  the  egg  (1st  Mat.  Sp.)  ;  Fig.  7,  the  first 
polar  body  (1st  PB)  formed  by  this  division;  Fig.  8,  the  sec- 
ond maturation  spindle  (2d  Mat.  Sp.)  and  the  sperm  nucleus 
and  spindle  (c£ZV) ;  Fig.  9,  the  division  of  the  male  and 
female  nuclei  in  the  first  cleavage  spindle  (1st  Cl.  Sp.).  (From 
F.  R.  Lillie.) 


22  HEREDITY  AND  ENVIRONMENT 

ment  such  as  thermal,  chemical  and  electrical 
changes — it  is,  in  short,  a  distinct  living  thing, 
an  individuality.  Under  proper  environ- 
mental conditions  this  fertilized  egg  cell  de- 
velops, step  by  step,  without  the  addition  of 
anything  from  the  outside  except  food,  water, 
oxygen,  and  such  other  raw  materials  as 
are  necessary  to  the  life  of  any  adult  animal, 
into  the  immensely  complex  body  of  a  star-fish, 
a  frog,  or  a  man.  At  the  same  time,  from  the 
relatively  simple  reactions  and  activities  of  the 
fertilized  egg  there  develop,  step  by  step, 
without  the  addition  of  anything  from  without 
except  raw  materials  and  environmental 
stimuli,  the  multifarious  activities,  reactions, 
instincts,  habits,  and  intelligence  of  the  ma- 
ture animal. 

Is  not  this  miracle  of  development  more 
wonderful  than  any  possible  miracle  of  cre- 
ation? And  yet  as  one  watches  this  marvellous 
process  by  which  the  fertilized  egg  grows  into 
the  embryo,  and  this  into  the  adult,  each  step 
appears  relatively  simple,  each  perceptible 
change  is  minute;  but  the  changes  are  in- 
numerable and  unceasing  and  in  the  end  they 


FACTS  AND  FACTORS  OF  DEVELOPMENT 


accomplish  this  miracle  of  transforming  the 
fertilized  egg  cell  into  the  fish,  or  frog,  or 
man — a  thing  which  would  be  incredible  were 
it  not  for  the  fact  that  it  has  been  seen  by 
hundreds  of  observers  and  can  be  verified  at 
any  time  by  those  who  will  take  the  trouble  to 
study  the  process  for  themselves. 
A 


FIG.  10.     FOUR  STAGES  IN  THE  CLEAVAGE  OF  THE  EGG  OF  THE 
SHEEP;  pb,  polar  bodies.     (After  Assheton.) 


24  HEREDITY  AND  ENVIRONMENT 

3.  Cleavage. — When  the  two  germ  nuclei, 
egg  nucleus  and  sperm  nucleus,  have  come 
into  contact  after  the  fertilization  of  the  egg 
they  divide  by  a  complicated  process  known  as 
mitosis,  or  indirect  nuclear  division  (Fig.  9). 
The  centrosome,  which  usually  accompanies 
the  sperm  nucleus  in  its  passage  through  the 
egg,  divides  and  forms  a  spindle-shaped  figure 
with  astral  radiations  at  its  two  poles  (Figs. 
7,  8).  The  chromatin,  or  stainable  substance 
of  the  nucleus,  takes  the  form  of  threads,  the 
chromosomes  ( Fig.  9 ) ,  of  which  there  is  a  con- 
stant number  for  each  species  of  animal  and 
plant.  Each  chromosome  then  splits  length- 
wise, its  two  halves  moving  to  opposite  ends 
of  the  spindle,  in  which  position  the  daughter 
chromosomes  fuse  together  to  form  the  daugh- 
ter nuclei.  In  this  way  the  chromatin  of  the 
egg  and  sperm  nuclei  is  exactly  halved. 

After  the  germ  nuclei  have  divided  in  this 
manner  the  entire  egg  divides  by  a  process  of 
constriction  into  two  cells  (Figs.  10, 11 ).  This 
is  the  beginning  of  a  long  series  of  cell  divi- 
sions, each  of  them  essentially  like  the  first,  by 
which  the  egg  is  subdivided  successively  into 


FIG.  11.  SUCCESSIVE  STAGES  IN  THE  CLEAVAGE  AND  GASTRULA- 
TION  OF  Amphioxus.  A,  one  cell;  B,  two  cells;  C  and  D,  four 
cells;  E,  eight  cells;  F,  sixteen  cells;  G,  blastula  stage  of  about 
ninety-six  cells;  H,  section  through  the  same  showing  the 
cleavage  cavity;  I,  blastula  seen  from  the  left  side  showing 
three  zones  of  cells,  viz.,  an  upper  clear  zone  of  ectoderm,  a 
middle  (faintly  shaded)  zone  of  mesoderm  and  a  lower  (deeply 
shaded)  zone  of  endoderm  cells;  J,  section  through  the  same 
showing  these  three  types  of  cells;  K  and  L,  successive  stages 
in  the  infolding  of  the  endoderm;  cells  indicated  as  in  the  pre- 
ceding figure.  In  all  figures  except  D  the  polar  body  is  shown 
at  the  upper  pole.  Figs.  A-H  after  Hatschek.  a,  anterior; 
p,  posterior;  v,  ventral;  d,  dorsal;  be,  blastocoel;  gc,  gastrocoel. 


26  HEREDITY  AND  ENVIRONMENT 

a  constantly  increasing  number  of  cells.  Dur- 
ing the  earlier  divisions  there  is  little  or  no  in- 
crease in  the  volume  of  the  egg,  consequently 
successive  generations  of  cells  continually 
grow  smaller  (Figs.  10,  11).  This  process 
is  known  as  the  cleavage  of  the  egg,  and 
by  it  the  egg  is  not  only  split  up  into  a  con- 
siderable number  of  small  cells,  but  a  much 
more  important  result  is  that  the  different 
kinds  of  protoplasm  in  the  egg  become  isolated 
in  different  cleavage  cells,  so  that  these  sub- 
stances can  no  longer  freely  commingle.  The 
cleavage  cells,  in  short,  come  to  contain  dif- 
ferent kinds  of  substance,  and  thus  to  differ 
from  one  another.  The  differentiations  of  the 
cleavage  cells  appear  much  earlier  in  some 
forms  than  in  others,  but  in  all  cases  such 
differentiations  appear  during  cleavage. 

4.  Embryogeny. — From  this  stage  onward 
the  course  of  development  differs  in  different 
classes  of  animals  to  such  an  extent  that  it  is 
difficult  to  formulate  any  general  description 
which  will  apply  to  all  of  them.  Usually  the 
many  cleavage  cells  form  a  hollow  sphere,  the 
blastula  (Fig.  11,  H),  and  this  in  turn  be- 


FACTS  AND  FACTORS  OF  DEVELOPMENT        27 

comes  a  gastrula  (Fig.  11,  K,  L),  in  which 
at  first  two,  and  later  three,  groups  or  layers 
of  cells  may  be  recognized;  the  outer  layer, 
which  is  formed  from  cells  nearest  the  upper 
pole  of  the  egg,  is  the  ectoderm;  the  inner 
layer,  or  endoderm,  is  formed  from  cells  near- 
est the  lower  pole;  a  middle  layer,  or  group 
of  cells,  the  mesoderm,  is  formed  from  cleav- 
age cells  which  in  vertebrates  lie  between  the 
upper  and  lower  poles. 


FIG.  12,  A  and  B.  Two  LATER  STAGES  IN  THE  DEVELOPMENT 
OF  Amphioxus,  showing  the  elongation  of  the  embryo  in  the 
antero-posterior  axis  (a  p),  and  formation  of  the  somites 
(som)  ;  neural  groove  (ng)  and  neural  tube  (nt) ;  ect,  ecto- 
derm; mes,  mesoderm;  ac,  alimentary  canal.  (After  Hatschek.) 


28  HEREDITY  AND  ENVIRONMENT 

5.  Organogeny. — By  further  differentiation 
of  the  cells  of  these  layers  and  by  dissimilar 
growth  and  folding  of  the  layers  themselves 
the  various  organs  of  the  embryo  begin  to  ap- 
pear. From  the  ectoderm  is  formed  the  outer 
layer  of  the  skin  and  the  nervous  system ;  from 
the  endoderm  arise  the  lining  of  the  aliment- 
ary canal  and  its  outgrowths;  from  the  meso- 
derm  come,  in  whole  or  in  part,  the  skeletal, 
muscular,  vascular,  excretory,  and  reproduc- 
tive systems.  In  vertebrates  the  nervous  sys- 
-tem  appears  as  a  plate  of  rather  large  ecto- 
derm cells  (Fig.  13  np)  ;  this  plate  rolls  up  at 
its  sides  to  form  a  groove  (Fig.  13  C)  and 
then  a  tube  (Fig.  13  D)  ;  and  by  enlarge- 
ment of  certain  portions  of  this  tube  and  by 
foldings  and  thickenings  of  its  walls  the  brain 
and  spinal  cord  are  formed  (Fig.  15,  C, 
D ) .  The  retina  or  sensory  portion  of  the  eye 
is  formed  as  an  outgrowth  from  the  fore  part 
of  the  brain  (Fig.  15,  D)  ;  the  sensory  portion 
of  the  ear  comes  from  a  cup-shaped  depres- 
sion of  the  superficial  ectoderm  which  covers 
the  hinder  portion  of  the  head  (Fig.  15,  E  and 
F) .  The  back-bone  begins  to  appear  as  a  deli- 


FACTS  AND  FACTORS  OF  DEVELOPMENT        29 

cate  cellular  rod  (Fig.  13,  ch),  which  then  in 
higher  vertebrates  becomes  surrounded  suc- 
cessively by  a  fibrous,  a  cartilaginous,  and  a 
bony  sheath.  And  so  one  might  go  on  with  a 
description  of  all  the  organs  of  the  body,  each 
of  which  begins  as  a  relatively  simple  group 
or  layer  of  cells,  which  gradually  become  more 
complicated  by  a  process  of  growth  and  dif- 


FIG.  13.  CROSS  SECTION  OF  Amphioxus  LARVAE  IN  SUCCESSIVE 
STAGES  OF  DEVELOPMENT.  A,  through  a  larva  similar  to  11A; 
B  and  C,  of  a  larva  similar  to  11B;  D,  of  a  still  older  larva; 
ect,  ectoderm;  ent,  endoderm;  mes,  mesoderm;  ch,  notochord; 
np,  neural  plate;  gc,  gastrocoel;  ac,  alimentary  canal;  coel, 
coelom. 


30  HEREDITY  AND  ENVIRONMENT 

ferentiation,  until  these  embryonic  organs  as- 
sume more  and  more  the  mature  form. 

6.  Oviparity  and  Viviparity. — This  very 
brief  and  general  statement  of  the  manner  of 
embryonic  development  applies  to  all  verte- 
brates, man  included.  There  are  many  special 
features  of  human  development  which  are 
treated  at  length  in  works  on  embryology,  but 
which  need  not  detain  us  here  since  they  do 
not  affect  the  general  principles  of  develop- 
ment already  outlined.  In  one  regard  the  de- 
velopment of  the  human  being  or  of  any  mam- 
mal is  apparently  very  different  from  that  of 
a  bird  or  frog  or  fish,  viz.,  in  the  fact  that  in 
the  former  the  embryonic  development  takes 
place  within  the  body  of  the  mother  whereas 
in  the  latter  the  eggs  are  laid  before  or  soon 
after  fertilization.  In  man,  after  the  cleavage 
of  the  egg,  a  hollow  vesicle  is  formed,  which 
becomes  attached  to  the  uterine  walls  by 
means  of  processes  or  villi  which  grow  out 
from  it  (Fig.  14,  D,  E,  F)  while  only  a  small 
portion  of  the  vesicle  becomes  transformed 
into  the  embryo.  There  is  thus  established  a 
connection  between  the  embryo  and  the  uterine 


FACTS  AND  FACTORS  OF  DEVELOPMENT        31 


FIG..  14.  DIAGRAMS  SHOWING  THE  EARLY  DEVELOPMENT  OF 
THE  HUMAN  OOSPEHM.  A,  cleavage  stage  which  has  just  come 
into  the  uterus;  B  and  C,  blastodermic  vesicles  embedded  in 
the  mucous  membrane  of  the  uterus;  D,  E  and  F,  longitudinal 
sections  of  later  stages,  the  anterior  and  posterior  poles  being 
marked  by  the  axis  a  p.  In  C  cavities  have  appeared  in  the 
ectoderm,  entoderm  and  mesoderm.  D,  villi  forming  from 
the  trophoblast  (nutritive  layer,  tr)  ;  black  indicates  ectoderm 
(ect)  ;  oblique  lines,  entoderm;  few  stipples,  mesoderm;  V,  villi; 
am,  amnion;  ys,  yolk  sac;  n,  neurenteric  canal;  x  25.  (After 
Keibel.) 


32  HEREDITY  AND  ENVIRONMENT 

walls  through  which  nutriment  is  absorbed  by 
the  embryo.  And  yet  this  difference  is  not 
a  fundamental  one  for  in  different  animals 
there  are  all  stages  of  transition  between  these 
two  modes  of  development.  While  in  most 
fishes,  amphibians  and  reptiles  the  eggs  are 
laid  at  the  beginning  of  development  and  are 
free  and  independent  during  the  whole  course 
of  ontogeny,  there  are  certain  species  in  each 
of  these  classes  in  which  the  development  takes 
place  within  the  body  of  the  mother.  Even 
in  birds  a  portion  of  the  development  takes 
place  within  the  body  of  the  female  before  the 
eggs  are  laid,  and  there  are  mammals  (mono- 
tremes)  which  lay  eggs,  while  in  others  (mar- 
supials) the  young  are  born  in  a  very  imperfect 
condition.  These  facts  indicate  that  there  is  no 
fundamental  difference  between  oviparity  and 
viviparity.  In  the  latter  the  union  between  the 
embryo  and  the  mother  is  a  nutritive  but  not  a 
protoplasmic  one.  Blood  plasma  passes  from 
one  to  the  other  by  a  process  of  soakage,  and 
the  only  material  influences  which  can  affect 
the  developing  embryo  are  such  as  may  be 
conveyed  through  the  blood  plasma  and  are 


FIG.  15.  A-H,  successive  stages  in  the  early  development  of 
the  human  embryo.  A,  blastodermic  vesicle  showing  primitive 
axis  in  embryonic  area;  age  unknown.  B,  blastodermic  vesicle 
attached  to  uterine  wall  at  the  posterior  pole,  showing  neural 
groove;  age  unknown.  C,  later  stage  in  which  the  neural  folds 
are  closing  and  five  pairs  of  somites  have  appeared;  age,  ten 
to  fourteen  days.  D,  stage  of  fourteen  somites  showing  en- 
largements of  the  neural  folds  at  the  anterior  end  which  will 
form  the  brain;  age,  fourteen  to  sixteen  days.  E  and  F  later 
stages,  the  latter  with  twenty-three  somites  and  three  visceral 
clefts.  The  ear  shows  as  a  depression  at  the  dorsal  angle  of 
the  second  cleft.  O,  embryo  of  thirty-five  somites  showing  eye, 
branchial  arches  and  limb  buds.  H,  embryo  of  thirty-six 
somites  showing  nasal  pit,  eye,  branchial  arches  and  clefts, 
limb  buds  and  heart.  (After  Keibel.) 


34  HEREDITY  AND  ENVIRONMENT 

chiefly  nutritive  in  character.  Careful  studies 
have  shown  that  supposed  "maternal  impres- 
sions" of  the  physical,  mental,  or  emotional 
conditions  of  the  mother  upon  the  unborn  child 
have  no  existence  in  fact,  except  in  so  far  as 
the  quality  of  the  mother's  blood  may  be 
changed  and  may  affect  the  child.  At  no  time, 
whether  before  or  after  birth,  is  the  mother 
more  than  nurse  to  the  child.  Hereditary  in- 
fluences are  transmitted  only  through  the  egg 
cell  and  the  sperm  cell  and  these  influences 
are  not  affected  by  intra-uterine  development. 
The  principles  of  heredity  and  development 
are  the  same  in  oviparous  and  in  viviparous 
animals — in  fishes,  frogs,  birds  and  men. 

Summary. — This  is  a  very  brief  and  incom- 
plete statement  of  some  of  the  important  stages 
or  phases  of  the  development  of  the  body  of 
man  or  of  any  other  vertebrate.  In  all  cases 
development  begins  with  the  fertilized  egg 
which  contains  none  of  the  structures  of  the  de- 
veloped animal,  though  it  may  exhibit  the 
polarity  and  symmetry  of  the  adult  and  may 
also  contain  specific  kinds  of  protoplasm  which 
will  give  rise  to  specific  tissues  or  organs  of 


FACTS  AND  FACTORS  OF  DEVELOPMENT        35 
A  B 


FIG.  16.  A,  human  embryo  of  forty-two  somites;  age  about 
twenty-one  days.  B,  embryo  of  about  four  weeks.  C,  still 
older  embryo  showing  the  beginnings  of  the  formation  of 
digits.  D,  embryo  of  about  two  months.  (After  Keibel.) 


36  HEREDITY  AND  ENVIRONMENT 

the  adult.  From  this  egg  cell  arise  by  divi- 
sion many  cells  which  differ  from  one  another 
more  and  more  as  development  proceeds,  until 
finally  the  adult  animal  results.  A  specific 
type  of  development  is  due  to  a  specific  organ- 
ization of  the  germ  cells  with  which  develop- 
ment begins,  but  the  earlier  differentiations 
of  the  egg  are  relatively  few  and  simple  as 
compared  with  the  bewildering  complexities 
of  the  adult,  and  the  best  way  of  understand- 
ing adult  structures  is  to  trace  them  back  in 
development  to  their  simpler  beginnings  and 
to  study  them  in  the  process  of  becoming. 

7.  Development  of  Functions. — The  de- 
velopment of  functions  goes  hand  in  hand 
with  the  development  of  structures;  indeed 
function  and  structure  are  merely  different 
aspects  of  one  and  the  same  thing,  namely 
organization.  All  the  general  functions  of 
living  things  are  present  in  the  germ  cells,  viz., 

(1)  Constructive  and  destructive  metabolism, 

(2)  Reproduction,  as  shown  in  the  division  of 
cells  and  cell  constituents,  (3)  Irritability,  or 
the  capacity  of  receiving  and  responding  to 
stimuli.    All  these  general  functions  of  living 


FACTS  AND  FACTORS  OF  DEVELOPMENT        37 

things  are  manifested  by  germ  cells,  but  as  de- 
velopment advances  each  of  these  functions  be- 
comes more  specialized,  more  complicated  and 
more  perfect.  A  cell  which  at  an  early  stage 
was  protective,  locomotor  and  sensory  in  func- 
tion may  give  rise  to  daughter  cells  in  which 
these  functions  are  distributed  to  different 
cells;  cells  which  at  an  early  stage  were  sensi- 
tive to  many  kinds  of  stimuli  give  rise  to 
daughter  cells  which  are  especially  sensitive  to 
one  particular  kind  of  stimulus,  such  as  vi- 
bration, light,  or  chemicals. 

Functions  develop  from  a  generalized  to  a 
specialized  condition  by  the  process  of  "physi- 
ological division  of  labor"  which  accompanies 
morphological  division  of  substance.  But  just 
as  in  the  development  of  structures  new  parts, 
which  were  not  present  in  the  germ,  appear  by 
a  process  of  "creative  synthesis,"  so  new  func- 
tions appear  in  the  course  of  development, 
which  are  not  merely  sorted  out  of  the  general 
functions  present  at  the  beginning,  but  whicli 
are  created  by  the  interaction  and  synthesis  of 
parts  and  functions  previously  present. 

Much  less  attention  has  been  paid  to  the  de- 


38  HEREDITY  AND  ENVIRONMENT 

velopment  of  functions  than  to  the  develop- 
ment of  structures,  and  consequently  it  is  not 
possible  to  describe  the  former  with  the  same 
degree  of  detail  as  the  latter.  But  in  spite  of 
the  lack  of  detailed  knowledge  regarding  the 
development  of  particular  functions  the  gen- 
eral fact  of  such  development  is  well  estab- 
lished. To  what  extent  structures  may  modify 
functions  or  functions  structures,  in  the  course 
of  development,  is  a  problem  which  has  been 
much  discussed,  and  upon  the  answer  to  it  de- 
pends the  fate  of  certain  important  theories, 
for  example  Lamarckism;  but  this  problem 
can  be  solved  only  by  thoroughgoing  experi- 
mental and  analytical  work.  In  the  meantime 
it  seems  safe  to  conclude  that  living  structures 
and  functions  are  inseparable  and  that  any- 
thing which  modifies  one  of  these  must  of 
necessity  modify  the  other  also;  they  are 
merely  different  aspects  of  organization,  and 
are  dealt  with  separately  by  the  morphologists 
and  physiologists  only  as  a  matter  of  conven- 
ience. At  the  same  time  there  can  be  no  doubt 
that  minute  changes  of  function  can  frequently 
be  detected  where  no  corresponding  change  of 


FACTS  AND  FACTORS  OF  DEVELOPMENT        39 

structure  can  be  seen,  but  this  shows  only  that 
physiological  tests  may  be  more  delicate  than 
morphological  ones.  In  certain  lines  of  mod- 
ern biological  work,  such  as  bacteriology, 
cytology,  and  genetics,  many  functional  dis- 
tinctions are  recognizable  between  organisms 
which  are  morphologically  indistinguishable. 
But  this  does  not  signify  that  functional 
changes  precede  structural  ones,  but  only  that 
the  latter  are  more  difficult  to  see  than  the 
former.  For  every  change  of  function  it  is 
probable  that  an  "unlimited  microscopist" 
could  discover  a  corresponding  change  of 
structure. 

II.  DEVELOPMENT  or  THE  MIND 

The  development  of  the  mind  parallels  that 
of  the  body:  whatever  the  ultimate  relations 
of  the  mind  and  body  may  be,  there  can  be  no 
reasonable  doubt  that  the  two  develop  together 
from  the  germ.  It  is  a  curious  fact  that  many 
people  who  are  seriously  disturbed  by  scientific 
teachings  as  to  the  evolution  or  gradual  de- 
velopment of  the  human  race  accept  with 
equanimity  the  universal  observation  as  to  the 


40  HEREDITY  AND  ENVIRONMENT 

development  of  the  human  individual, — mind 
as  well  as  body.  The  animal  ancestry  of  the 
race  is  surely  no  more  disturbing  to  philosophi- 
cal and  religious  beliefs  than  the  germinal 
origin  of  the  individual,  and  yet  the  latter  is  a 
fact  of  universal  observation  which  can  not  be 
relegated  to  the  domain  of  hypothesis  or 
theory,  and  which  can  not  be  successfully  de- 
nied. If  we  admit  the  fact  of  the  develop- 
ment of  the  entire  individual,  surely  it  matters 
little  to  our  philosophical  or  religious  beliefs 
to  admit  the  development  or  evolution  of  the 
race. 

The  origin  of  the  mind,  or  rather  of  the 
soul,  is  a  topic  upon  which  there  has  been  much 
speculation  by  philosophers  and  theologians. 
One  of  the  earliest  hypotheses  was  that  which 
is  known  as  transmigration  or  metempsychosis. 
This  doctrine  probably  reached  its  greatest 
development  in  ancient  India,  where  it  formed 
an  important  part  of  Buddhistic  belief;  it  was 
also  a  part  of  the  religion  of  ancient  Egypt; 
it  was  embodied  in  the  philosophies  of  Pytha- 
goras and  Plato.  According  to  these  teach- 
ings, the  number  of  souls  is  a  constant  one; 


FACTS  AND  FACTORS  OF  DEVELOPMENT        41 

souls  are  neither  made  nor  destroyed,  but  at 
birth  a  soul  which  had  once  tenanted  another 
body  enters  into  the  new  body.  This  doctrine 
was  generally  repudiated  by  the  Fathers  of 
the  Christian  Church.  Jerome  and  others 
adopted  the  view  that  God  creates  a  new  soul 
for  each  body  that  is  generated,  and  that  every 
soul  is  thus  a  special  divine  creation.  This  has 
become  the  prevailing  view  of  the  Christian 
Church  and  is  known  as  creationism.  On  the 
other  hand  Tertullian  taught  that  souls  of 
children  are  generated  from  the  souls  of  par- 
ents as  bodies  are  'from  bodies.  This  doctrine, 
which  is  known  as  traducianism,  has  been  de- 
fended by  certain  modern  theologians,  but  has 
been  formally  condemned  by  the  Roman 
Catholic  Church. 

Traducianism  undoubtedly  comes  nearer 
the  scientific  teachings  as  to  the  development 
of  the  mind  than  does  either  of  the  other  doc- 
trines named,  but  it  is  based  upon  the  preva- 
lent but  erroneous  belief  that  the  bodies  of  the 
parents  generate  the  body  of  the  child,  and 
that  correspondingly  the  souls  of  the  parents 
generate  the  soul  of  the  child.  Now  we  know 


42  HEREDITY  AND  ENVIRONMENT 

that  the  child  comes  from  germ  cells  which 
are  not  made  by  the  bodies  of  the  parents 
but  have  arisen  by  the  division  of  antecedent 
germ  cells.  Every  cell  comes  from  a  pre- 
existing cell  by  a  process  of  division,  and  every 
germ  cell  comes  from  a  pre-existing  germ  cell. 
Consequently  it  is  not  possible  to  hold  that  the 
body  generates  germ  cells,  nor  that  the  soul 
generates  souls.  The  only  possible  scientific 
position  is  that  the  mind  (or  soul)  as  well  as 
the  body  develops  from  the  germ. 

No  fact  in  human  experience  is  more  certain 
than  that  the  mind  develops  by  gradual  and 
natural  processes  from  a  simple  condition 
which  can  scarcely  be  called  mind  at  all;  no 
fact  in  human  experience  is  fraught  with 
greater  practical  and  philosophical  significance 
than  this,  and  yet  no  fact  is  more  generally 
disregarded.  We  know  that  the  greatest  men 
of  the  race  were  once  babies,  embryos,  germ 
cells,  and  that  the  greatest  minds  in  human 
history  were  once  the  minds  of  babies,  em- 
bryos and  germ  cells,  and  yet  this  stupendous 
fact  has  had  but  little  influence  on  our  beliefs 
as  to  the  nature  of  man  and  of  mind.  We 


FACTS  AND  FACTORS  OF  DEVELOPMENT        43 

rarely  think  of  Plato  and  Aristotle,  of  Shake- 
speare and  Newton,  of  Pasteur  and  Darwin, 
except  in  their  full  epiphany,  and  yet  we  know 
that  when  each  of  these  was  a  child  he  "thought 
as  a  child  and  spake  as  a  child,"  and  when 
he  was  a  germ  cell  he  behaved  as  a  germ  cell. 

The  development  of  the  mind  from  the  ac- 
tivities of  the  germ  cells  is  certainly  most  won- 
derful and  mysterious,  but  probably  no  more 
so  than  the  development  of  the  complicated 
body  of  the  adult  animal  from  the  structures 
of  the  germ.  Both  belong  to  the  same  order 
of  phenomena  and  there  is  no  more  reason  for 
supposing  that  the  mind  is  supernaturally 
created  than  that  the  body  is.  Indeed,  we 
know  that  the  mind  is  formed  by  a  process  of 
development,  and  the  stages  of  this  develop- 
ment are  fairly  well  known.  There  is  nowhere 
in  the  entire  course  of  mental  development  a 
sudden  appearance  of  psychical  processes,  but 
rather  a  gradual  development  of  these  from 
simpler  and  simpler  beginnings.  No  detailed 
study  has  been  made  of  the  reactions  of  human 
germ  cells  and  embryos,  but  there  is  every 
reason  to  believe  that  these  reactions  are 


44  HEREDITY  AND   ENVIRONMENT 

simpler  in  the  embryo  and  germ  cell  than  in 
the  infant,  and  that  they  are  generally  similar 
to  the  reactions  of  the  germ  cells  and  embryos 
of  other  animals,  and  to  the  behavior  of  many 
lower  organisms. 

A  few  years  ago  such  a  statement  would 
have  been  branded  as  "materialism"  and 
promptly  rejected  without  examination  by 
those  who  are  frightened  by  names.  But  the 
general  spread  of  the  scientific  spirit  is  shown 
not  only  by  the  growing  regard  for  evidence 
but  also  by  the  decreasing  power  of  epithets. 
"Materialism,"  like  many  another  ghost,  fades 
away  into  thin  air  or  at  least  loses  many  of 
its  terrors,  when  closely  scrutinized.  But  the 
statement  that  mind  develops  from  the  germ 
cells  is  not  an  affirmation  of  materialism,  for 
while  it  identifies  the  origin  of  the  entire  indi- 
vidual, mind  and  body,  with  the  development 
of  the  germ,  it  does  not  assert  that  "matter" 
is  the  cause  of  "mind"  either  in  the  germ  or 
in  the  adult.  It  must  not  be  forgotten  that 
germ  cells  are  living  things  and  that  we  go  no 
further  in  associating  the  beginnings  of  mind 
with  the  beginnings  of  body  in  the  germ  than 


FACTS  AND  FACTORS  OF  DEVELOPMENT        45 

we  do  in  associating  mind  and  body  in  the 
adult.  It  is  just  as  materialistic  to  hold  that 
the  mind  of  the  mature  man  is  associated  with 
his  body  as  it  is  to  hold  that  the  beginnings 
of  mind  in  the  germ  are  associated  with  the 
beginnings  of  the  body,  and  both  of  these 
tenets  are  incontrovertible. 

It  seems  to  me  that  the  mind  is  related  to 
the  body  as  function  is  to  structure;  there  are 
those  who  maintain  that  structure  is  the  cause 
of  function,  that  the  real  problem  in  evolution 
or  development  is  the  transformation  of  one 
structure  into  another,  and  that  the  functions 
which  go  with  certain  structures  are  merely 
incidental  results ;  on  the  other  hand  are  those 
who  maintain  that  function  is  the  cause  of 
structure  and  that  the  problem  of  evolution 
or  development  is  the  change  which  takes  place 
in  functions  and  habits,  these  changes  causing 
corresponding  transformations  of  structure. 
Among  adherents  of  the  former  view  may  be 
classed  many  morphologists  and  Neo-Darwin- 
ians;  among  proponents  of  the  latter,  many 
physiologists  and  Neo-JLamarckians.  It  seems 
to  me  that  the  defenders  of  each  of  these  views 


46  HEREDITY   AND  ENVIRONMENT 

fail  to  recognize  the  essential  unity  of  the  en- 
tire organism,  structure  as  well  as  function; 
that  neither  of  these  is  the  cause  of  the  other, 
though  each  may  modify  or  condition  the  other, 
but  that  they  are  two  aspects  of  one  common 
thing,  viz.,  organization.  In  the  same  way  I 
think  that  the  body  or  brain  is  not  the  cause 
of  mind,  nor  mind  the  cause  of  body  or  brain, 
but  that  both  are  inherent  in  one  common  or- 
ganization or  individuality. 

In  asserting  that  the  mind  develops  from 
the  germ  as  the  body  does,  no  attempt  is  made 
to  explain  the  fundamental  properties  of  body 
or  mind.  As  the  structures  of  the  body  may 
be  traced  back  to  certain  fundamental  struc- 
tures of  the  germ  cell,  so  the  characteristics  of 
the  mind  may  be  traced  back  to  certain  funda- 
mental properties  and  activities  of  the  germ. 
Many  of  the  psychical  processes  may  be  traced 
back  in  their  development  to  properties  of 
sensitivity,  reflex  motions,  and  persistence  of 
the  effects  of  stimuli.  All  organisms  manifest 
these  properties  and  for  aught  we  know  to 
the  contrary  they  may  be  original  and  neces- 
sary characteristics  of  living  things.  In  the 


FACTS  AND  FACTORS  OF  DEVELOPMENT        47 

simplest  protoplasm  we  find  organization,  that 
is,  structure  and  function,  and  in  germinal 
protoplasm  we  find  the  elements  of  the  mind 
as  well  as  of  the  body,  and  the  problem  of  the 
ultimate  relation  of  the  two  is  the  same 
whether  we  consider  the  organism  in  its  germi- 
nal or  in  its  adult  stage. 

In  some  way  the  mind  as  well  as  the  body 
develops  out  of  the  germ.  What  are  the 
germinal  bases  of  mind?  What  are  the  psy- 
chical Anlagen  in  embryos  and  how  do  they 
develop?  In  this  case,  even  more  than  in  the 
development  of  the  body,  we  are  compelled  to 
rely  upon  the  comparison  of  human  develop- 
ment with  that  of  other  animals,  but  the  great 
principle  of  the  oneness  of  life,  as  respects  its 
fundamental  processes,  has  never  yet  failed 
to  hold  true  and  will  not  fail  us  here.  In  the 
study  of  the  psychical  processes  of  organisms 
other  than  ourselves  we  are  compelled  to  rely 
upon  a  study  of  their  activities,  their  reactions 
to  stimuli,  since  we  can  not  approach  the  sub- 
ject in  any  other  way.  The  reactions  and  be- 
havior of  organisms  under  normal  and  experi- 
mental conditions  give  the  only  insight  which 


48  HEREDITY  AND  ENVIRONMENT 

we  can  get  into  their  psychical  processes; 
and  this  applies  to  men  no  less  than  to 
protozoa. 

1.  Sensitivity. — The  most  fundamental 
phenomenon  in  the  behavior  of  organisms  is 
irritability  or  sensitivity,  which  is  the  capacity 
of  receiving  and  responding  to  stimuli:  this  is 
one  of  the  fundamental  properties  of  all  proto- 
plasm. But  living  matter  is  not  equally  sensi- 
tive to  all  stimuli,  nor  to  all  strengths  of  the 
same  stimulus.  Many  of  the  simplest  unicel- 
lular plants  and  animals  show  that  they  are 
differentially  sensitive;  they  often  move  to- 
ward weak  light  and  away  from  strong  light, 
away  from  extremes  of  heat  and  cold,  into 
certain  chemical  substances  and  away  from 
others;  in  short,  all  organisms,  even  the  sim- 
plest, may  respond  differently  to  different 
kinds  of  stimuli  or  to  different  degrees  of  the 
same  stimulus.  This  is  what  is  known  as  dif- 
ferential sensitivity  (Figs.  17, 18, 19) .  On  the 
other  hand,  many  organisms  respond  in  the 
same  way  to  different  stimuli,  and  this  may  be 
taken  to  indicate  generally  that  they  are  not 
differentially  sensitive  to  such  stimuli;  it  is 


FACTS  AND  FACTORS  OF  DEVELOPMENT        49 


« B  C          D  E  »  F  9 

FIG.  17.  DISTRIBUTION  OF  BACTERIA  \~$  THE  SPECTRUM.  The 
largest  group  is  in  the  ultra- red  at  the  left;  the  next  largest 
group  is  in  the  yellow-orange  close  to  the  line  D.  (From  Jen- 
nings, after  Engelmann.) 

not  to  be  concluded  because  organisms  respond 
differently  to  certain  stimuli  that  they  are 
therefore  capable  of  distinguishing  between 
all  kinds  of  stimuli,  for  this  is  certainly  not 
true.  Even  in  adult  men  the  capacity  of  dis- 
tinguishing between  different  kinds  of  stimuli 
is  far  from  perfect. 

Egg  cells  and  spermatozoa  show  this  prop- 
erty of  sensitivity.  The  egg  is  generally  in- 
capable of  locomotion,  and  since  the  results  of 
stimulation  must  usually  be  detected  by  move- 
ments it  is  not  easy  to  determine  to  what  ex- 
tent the  egg  is  sensitive;  but  though  the  egg 
lacks  the  power  of  locomotion,  it  possesses  in 
a  marked  degree  the  power  of  intra-cellular 
movement  of  the  cell  contents.  When  a 
spermatozoon  comes  into  contact  with  the  sur- 


50  HEREDITY  AND  ENVIRONMENT 

face  of  the  egg  the  cortical  protoplasm  of  the 
egg  flows  toward  that  point  and  may  form  a 
cone  or  protoplasmic  prominence  into  which 
the  sperm  is  received  (Figs.  4,  5,  EC).  It  is 
an  interesting  fact  that  the  same  sort  of  re- 
sponse follows  when  a  frog's  egg  is  pricked 
by  a  needle,  thus  showing  that  in  this  case  the 
egg  does  not  distinguish  between  the  prick  of 
the  needle  and  that  of  the  spermatozoon.  The 
spermatozoon  is  usually  a  locomotor  cell  and 
it  responds  differently  to  certain  stimuli,  just 
as  many  bacteria  and  protozoa  do;  sper- 
matozoa are  strongly  stimulated  by  weak 
alkali  and  alcohol,  they  gather  in  certain 
chemical  substances  and  not  in  others,  they 
collect  in  great  numbers  around  fertilizable 
egg  cells,  etc. 

The  movements  of  fertilized  egg  cells, 
cleavage  cells,  and  early  embryonic  cells  are 
usually  limited  to  flowing  movements  within 
the  individual  cells.  These  movements,  which 
are  of  a  complicated  nature,  are  of  the  greatest 
significance  in  the  differentiation  of  the  egg 
into  the  embryo ;  they  are  caused  chiefly  by  in- 
ternal stimuli  and  by  non-localized  external 


FACTS  AND  FACTORS  OF  DEVELOPMENT        51 


FIG.  18,  a,  b,  c.  REPULSION  OF  Spirilla  BY  COMMON  SALT. 
a,  condition  immediately  after  adding  crystals;  b  and  c,  later 
stages  in  the  reaction. 

x,  y,  z,  repulsion  of  Spirilla  by  distilled  water.  The  upper 
drop  consists  of  sea-water  containing  Spirilla,  the  lower  drop, 
of  distilled  water.  At  x  these  have  just  been  united  by  a 
narrow  neck;  at  y  and  z,  the  bacteria  have  retreated  before  the 
distilled  water.  (From  Jennings,  after  Massart.) 


52  HEREDITY  AND  ENVIRONMENT 

ones.  Modifications  of  the  external  stimuli 
often  lead  to  modifications  of  these  intra- 
cellular  movements  and  to  abnormal  types  of 
cleavage  and  development — in  short,  these 
movements  show  that  the  fertilized  egg  is  dif- 
ferentially sensitive. 

In  the  further  course  of  development 
particular  portions  of  the  embryo  become  es- 
pecially sensitive  to  some  kinds  of  stimuli, 
while  other  portions  become  sensitive  to  oth- 
ers. In  this  way  the  different  sense  organs, 
each  especially  sensitive  to  one  particular 
kind  of  stimulus,  arise  from  the  generalized 
sensitivity  of  the  oosperm,  and  thus  general 
sensitivity,  which  is  a  property  of  all  proto- 
plasm, becomes  differential  sensitivity  and 
special  senses  in  the  process  of  embryonic  dif- 
ferentiation. Such  sensitivity  is  the  basis  of 
all  psychic  processes;  sensations  are  the  ele- 
ments of  the  mind. 

2.  Tropisms,  Reflexes,  Instincts. — All  the 
responses  of  germ  cells,  and  of  the  simplest 
organisms,  to  stimuli  are  in  the  nature  of  trop- 
isms  or  reflexes,  that  is,  relatively  simple, 
automatic  responses.  Such  tropisms  or  re- 


FACTS  AND  FACTORS  OF  DEVELOPMENT        53 

flexes  are  seen  in  the  movements  of  bacteria, 
protozoa  and  many  higher  animals  and  plants 
as  well  as  in  movements  of  spermatozoa,  the 
movements  of  the  protoplasm  in  egg  cells  and 
embryonic  cells,  the  movements  of  cells  and 


19*  19* 


26*  f 38 


10  °  25~ 

FIG.  19.  REACTIONS  OF  Paramecium  TO  HEAT  AND  COLD.  At 
a  the  infusoria  are  uniformly  distributed  in  a  trough,  both 
ends  of  which  have  a  temperature  of  19°;  at  6  the  infusoria 
are  shown  collected  at  the  cooler  end  of  the  trough;  at  c  they 
have  collected  at  the  warmer  end  of  the  trough.  (From  Jen- 
nings, after  Mendelssohn.) 


54  HEREDITY  AND  ENVIRONMENT 

cell  masses  in  the  formation  of  the  gastrula, 
alimentary  canal,  nervous  system  and  other 
organs.  Indeed  the  entire  process  of  develop- 
ment, whether  accompanied  by  visible  move- 
ments or  not,  may  be  regarded  as  a  series  of 
automatic  responses  to  stimuli. 

When  the  embryo  becomes  differentiated 
to  such  an  extent  as  to  have  specialized  organs 
for  producing  movement  its  capacity  for  mak- 
ing responsive  movements  to  stimuli  becomes 
much  increased.  If  the  responses  of  animals 
and  plants  to  stimuli  are  of  such  a  sort  that 
the  organism  turns  or  moves  toward  or  away 
from  a  source  of  stimulus  they  are  termed 
tropisms;  if  the  responses  are  very  compli- 
cated, one  response  calling  forth  another  and 
involving  many  reflexes,  as  is  frequently  the 
case  in  animals,  they  are  known  as  instincts. 
In  the  embryo  the  rhythmic  contractions  of 
heart,  amnion  and  intestine  are  early  manifes- 
tations of  reflex  motions.  These  appear  chiefly 
in  the  involuntary  muscles  before  nervous  con- 
nections are  formed,  the  protoplasm  of  the 
muscle  cells  probably  responding  directly  to 
the  chemical  stimulus  of  certain  salts  in  the 


FACTS  AND  FACTORS  OF  DEVELOPMENT        55 

body  fluids,  as  Loeb  has  shown  in  other  cases. 
Reflexes  which  appear  later  are  the  random 
movements  of  the  voluntary  muscles  of  limbs 
and  body,  which  are  called  forth  by  nerve  im- 
pulses. Tropisms  are  manifested  only  by  or- 
ganisms capable  of  considerable  free  movement 
and  hence  are  absent  in  the  foetus  though 
present  in  many  free  living  larvae  Some  in- 
stincts are  present  immediately  after  birth, 
such  as  the  instinct  of  sucking  or  crying, 
though  these  are  so  simple  when  compared 
with  some  instincts  which  develop  later  that 
they  might  be  classed  as  reflexes ;  it  is  doubtful 
whether  any  of  the  activities  before  birth  could 
properly  be  designated  as  instincts.  Reflexes, 
tropisms  and  instincts  have  had  a  phylogenetic 
as  well  as  an  ontogenetic  origin,  and  conse- 
quently we  might  expect  that  they  would  in 
general  make  for  the  preservation  of  the 
species,  and  as  a  matter  of  fact  we  usually  find 
that  they  are  remarkably  adapted  to  this  end. 
For  instance  the  instincts  of  the  human  infant 
to  grasp  objects,  to  suck  things  which  it  can 
get  into  its  mouth,  to  cry  when  in  pain,  are 
complicated  reflexes  which  have  survived  in 


56  HEREDITY  AND  ENVIRONMENT 

the  course  of  evolution  probably  because  they 
serve  a  useful  purpose. 

Very  much  has  been  written  on  the  nature 
and  origin  of  instincts,  but  the  best  available 
evidence  strongly  favors  the  view  that  instincts 
are  complex  reflexes,  which,  like  the  structures 
of  an  organism,  have  been  built  up,  both  onto- 
genetically  and  phylogenetically,  under  the 
stress  of  the  elimination  of  the  unfit,  so  that 
they  are  usually  adaptive. 

3.  Memory. — Another  general  character- 
istic of  protoplasm  is  the  capacity  of  storing 
up  or  registering  the  effects  of  previous 
stimuli.  A  single  stimulus  may  produce 
changes  in  an  organism  which  persist  for  a 
longer  or  shorter  time,  and  if  a  second  stimulus 
occurs  while  the  effect  of  a  previous  one  still 
persists,  the  response  to  the  second  stimulus 
may  be  very  different  from  that  to  the  first. 
Macf arlane  found  that  if  the  sensitive  hairs  on 
the  leaf  of  Dionaea,  the  Venus  fly-trap  (Fig. 
20,  SH),  be  stroked  once  no  visible  response 
is  called  forth,  but  if  they  be  stroked  a  second 
time  within  three  minutes  the  leaf  instantly 
closes.  If  a  longer  period  than  three  minutes 


FACTS  AND  FACTORS  OF  DEVELOPMENT         57 

elapses  after  the  first  stimulus  and  before  the 
second  no  visible  response  follows,  i.  e.}  two 
successive  stimuli  are  necessary  to  cause  the 
leaves  to  close,  and  the  two  must  not  be  more 
than  three  minutes  apart;  the  effects  of  the 
first  stimulus  are  in  some  way  stored  or  regis- 
tered in  the  leaf  for  this  brief  time.  This  kind 
of  phenomenon  is  widespread  among  living 
things  and  is  known  as  "summation  of  stim- 
uli." In  all  such  cases  the  effects  of  a  former 
stimulus  are  in  some  way  stored  up  for  a 
longer  or  shorter  time  in  the  protoplasm.  It 
is  possible  that  this  is  the  result  of  the  forma- 
tion of  some  chemical  substance  which  remains 
in  the  protoplasm  for  a  certain  time,  during 
which  time  the  effects  of  the  stimulus  are  said 
to  persist,  or  it  may  be  due  to  some  physical 
change  in  the  protoplasm  analogous  to  the 
"set"  in  metals  which  have  been  subjected  to 
mechanical  strain. 

Probably  of  a  similar  character  is  the  per- 
sistence of  the  effects  of  repeated  stimuli  and 
responses  on  any  organ  of  a  higher  animal.  A 
muscle  which  has  contracted  many  times  in  a 
definite  way  ultimately  becomes  "trained"  so 


58 


HEREDITY  AND  ENVIRONMENT 


that  it  responds  more  rapidly  and  more  ac- 
curately than  an  untrained  muscle;  and  the 
nervous  mechanism  through  which  the  stimu- 
lus is  transmitted  also  becomes  trained  in  the 
same  way.  Indeed  such  training  is  probably 
chiefly  a  training  of  the  nervous  mechanism. 
The  skill  of  the  pianist,  of  the  tennis  player, 
of  the  person  who  has  learned  the  difficult  art 


FIG.  20.  Dionaea  muscipula  (VENUS'  FLY-THAP).  Three 
leaves  showing  marginal  teeth  and  sensitive  hairs  (SH).  The 
leaf  at  the  left  is  fully  expanded,  the  one  at  the  right  is  closed. 


PACTS  AND  FACTORS  OF  DEVELOPMENT        59 

of  standing  and  walking,  or  the  still  more  diffi- 
cult art  of  talking,  is  probably  due  to  the  per- 
sistence in  muscles  and  nerves  of  the  effects 
of  many  previous  activities.  All  such  phe- 
nomena were  called  by  Hering  "organic 
memory,"  to  indicate  that  this  persistence  of 
the  effects  of  previous  activities  in  muscles  and 
other  organs  is  akin  to  that  persistence  of  the 
effects  of  previous  experiences  in  the  nervous 
mechanism  which  we  commonly  call  memory. 
It  seems  probable  that  this  ability  of  proto- 
plasm in  general  to  preserve  for  a  time  the 
effects  of  former  stimuli  is  fundamentally  of 
the  same  nature  as  the  much  greater  power  of 
nerve  cells  to  preserve  such  effects  for  much 
longer  periods  and  in  complex  associations,  a 
faculty  which  is  known  as  associative  memory. 
The  embryos,  and  indeed  even  the  germ  cells 
of  higher  animals,  may  safely  be  assumed  to 
be  endowed  with  protoplasmic  and  organic 
memory,  out  of  which,  in  all  probability,  de- 
velop associative  and  conscious  memory  in  the 
mature  organism. 

4.  Intellect,    Reason. — Even    the    intellect 
and  reason  which  so  strongly  characterize  man 


60  HEREDITY  AND  ENVIRONMENT 

have  had  a  development  from  relatively  simple 
beginnings.  All  children  come  gradually  to 
an  age  of  intelligence  and  reason.  In  its 
simpler  forms  at  least  reason  may  be  defined 
as  the  power  of  predicting  future  events  and 
of  reaching  conclusions  regarding  unexperi- 
enced phenomena  under  the  influence  of  past 
experience.  In  the  absence  of  individual  ex- 
perience young  children  have  none  of  this 
power,  but  it  comes  gradually  as  a  result  of 
remembering  past  experiences  and  of  fitting 
such  experiences  into  new  conditions.  Young 
infants  and  many  lower  animals  lack  the 
power  of  reason,  though  their  behavior  is  fre- 
quently of  such  a  sort  as  to  suggest  that  they 
are  reasoning.  Even  the  lowest  animals  avoid 
injurious  substances  and  conditions  and  find 
beneficial  ones;  more  complex  animals  learn 
to  move  objects,  solve  problems,  and  find  their 
way  through  labyrinths  in  the  shortest  and 
most  economical  way;  but  this  apparently  in- 
telligent and  purposive  behavior  has  been 
shown  to  be  due  to  the  general  elimination 
of  all  sorts  of  useless  activities,  and  to  the  per- 
sistence of  the  useful  ones. 


FACTS  AND  FACTORS  OF  DEVELOPMENT    61 

The  ciliated  infusorian,  Paramedum,  moves 
by  the  beating  of  cilia  which  are  arranged  in 
such  a  way  that  they  drive  the  animal  forward 
in  a  spiral  course.  However,  when  it  is 
strongly  irritated,  the  normal  forward  move- 
ment is  reversed ;  the  cilia  beat  forward  instead 
of  backward  and  the  animal  is  driven  back- 
ward for  some  distance  (Fig.  21,  1,  2,  3) ;  it 
then  stands  nearly  still,  merely  rolling  over 
and  swerving  toward  the  aboral  side,  and 
finally  it  goes  ahead  again,  usually  on  a  new 
course  (Fig.  21,  3,  4,  5,  6).  These  move- 
ments seem  to  be  conditioned  rather  rigidly  by 
the  organization  of  the  animal:  they  are  more 
or  less  fixed  and  mechanical  in  character, 
though  to  a  certain  extent  they  may  be  modi- 
fied by  experience  or  physiological  states. 
Paramedum  behaves  as  it  does  in  virtue  of 
its  constitution,  just  as  an  egg  develops  in  a 
particular  way  because  of  its  particular 
organization. 

But  although  limited  in  its  behavior  to  these 
relatively  simple  motor  reactions,  Paramedum 
does  many  things  which  seem  to  show  intelli- 
gence and  purpose.  It  avoids  many  injurious 


62 


HEREDITY  AND  ENVIRONMENT 


substances,  such  as  strong  salts  or  acids,  and  it 
collects  in  non-injurious  or  beneficial  sub- 
stances, such  as  weak  acids,  masses  of  bacteria 
upon  which  it  feeds,  etc.  It  avoids  extremes 
of  heat  and  cold  and  if  one  end  of  a  dish  con- 
taining Paramecia  is  heated  and  the  other  end 
is  cooled  by  ice,  the  Paramecia  collect  in  the 
region  somewhere  between  these  two  extremes 
(Fig.  19).  Jennings,  by  studying  carefully 
the  behavior  of  single  individuals,  established 
the  fact  that  this  apparently  intelligent  action 
is  due  to  differential  sensitivity  and  to  the 
single  motor  reaction  of  the  animal.  If  in  the 


FIG.  21.  DIAGRAM  OF  THE  AVOIDING  REACTION  OF  Parame- 
cium.  A  is  a  solid  object  or  other  source  of  stimulation.  1-6, 
successive  positions  occupied  by  the  animal.  The  rotation  on 
the  long  axis  is  not  shown.  (After  Jennings.) 


FACTS  AND  FACTORS  OF  DEVELOPMENT        63 

course  of  its  swimming  a  Paramecium  comes 
into  contact  with  an  irritating  substance  or 
condition,  it  backs  a  short  distance,  swerves 
toward  its  aboral  side,  and  goes  ahead  in  a 
new  path ;  if  it  again  comes  in  contact  with  the 
irritating  conditions  this  reaction  is  repeated, 
and  so  on  indefinitely  until  finally  a  path  is 
found  in  which  the  source  of  irritation  is 
avoided  altogether.  In  short,  Paramecium 
continually  tries  its  environment,  and  backs 
away  from  irritating  substances  or  conditions. 
Its  apparently  intelligent  reactions  are  thus 
explained  as  due  to  a  process  of  "trial  and 
error."1 

The  behavior  of  worms,  star-fishes,  crusta- 
ceans, mollusks,  as  well  as  of  fishes,  frogs, 


1  In  Paramecium,  there  is  certainly  no  consciousness  of  trial 
and  error,  and  probably  no  unconscious  attempt  on  the  part 
of  the  animal  to  attain  certain  ends.  Its  responses  are  reflexes 
or  tropisms,  which  are  determined  by  the  nature  of  the  animal 
and  the  character  of  the  stimulus.  The  fact  that  these  re- 
sponses are  in  the  main  self-preservative  is  due  to  the  teleo- 
logical  organization  of  Paramecium  which  has  been  evolved, 
according  to  current  opinion,  as  the  result  of  long  ages  of 
the  elimination  of  the  unfit.  If,  in  the  opinion  of  any  one, 
the  expression  "trial  and  error"  necessarily  involves  a  striving 
after  ends,  it  would  be  advisable  to  replace  it  in  this  case  by 
some  such  term  as  "useful  or  adaptive  reactions." 


64  HEREDITY  AND  ENVIRONMENT 

reptiles,  birds  and  mammals,  has  been  studied 
and  in  all  cases  it  is  found  that  their  method 
of  responding  to  stimuli  is  not  at  first  really 
purposive  and  intelligent  but  by  the  gradual 
elimination  of  useless  responses  and  the 
preservation  (or  remembering)  of  useful  ones 
the  behavior  may  come  to  be  purposive  and 
intelligent. 

Thorndike  found  that  when  dogs,  cats  and 
monkeys  were  confined  in  cages  which  could 
be  opened  from  the  inside  by  turning  a  button, 
or  pressing  upon  a  lever,  or  pulling  a  cord, 
they  at  first  clawed  around  all  sides  of  the  cage 
until  by  chance  they  happened  to  operate  the 
mechanism  which  opened  the  door.  There- 
after they  gradually  learned  by  experience, 
that  is,  by  trial  and  error,  and  finally  by  trial 
and  success,  just  where  and  how  to  claw  in 
order  to  get  out  at  once.  When  a  dog  has 
learned  to  turn  a  button  at  once  and  open  a 
door  we  say  he  is  intelligent,  and  if  he  can 
learn  to  apply  his  knowledge  of  any  particular 
cage  to  other  and  different  cages,  a  thing 
which  Thorndike  denies,  we  should  be  justified 
in  saying  that  he  reasons,  though  in  this  case 


FACTS  AND  FACTORS  OF  DEVELOPMENT        65 

intelligence  and  reason  are  founded  upon 
memory  of  many  past  experiences,  of  many 
trials  and  errors  and  of  a  few  trials  and 
successes. 

There  is  every  evidence  that  human  beings 
arrive  at  intelligence  and  reason  by  the  same 
process,  a  process  of  many  trials  and  errors 
and  a  few  trials  and  successes,  a  remembering 
of  these  past  experiences  and  an  application 
of  them  to  new  conditions.  A  baby  grasps  for 
things  which  are  out  of  its  reach,  until  it  has 
learned  by  experience  to  appreciate  distances; 
it  tests  all  sorts  of  pleasant  and  unpleasant 
things  until  it  has  learned  to  avoid  the  latter 
and  seek  the  former;  it  experiments  with  its 
own  body  until  it  has  learned  what  it  can  do 
and  what  it  can  not  do.  Is  not  this  learning 
by  experience  akin  to  the  same  process  in  the 
dog  and  more  remotely  to  the  trial  and  error 
of  the  earthworm  or  the  adaptive  reflexes  of 
Paramecium?  Is  not  intelligence  and  reason 
in  all  of  us,  and  upon  all  subjects,  based  upon 
the  same  processes  of  trial  and  error,  memory 
of  past  experiences  and  application  of  this  to 
new  conditions?  Surely  this  is  true  in  all  ex- 


66  HEREDITY  AND  ENVIRONMENT 

perimental  and  scientific  work.  Indeed  the 
scientific  method  is  the  method  of  trial  and 
error,  and  finally  trial  and  success — the 
method  recommended  by  St.  Paul  to  prove  all 
things  and  hold  fast  that  which  is  good. 

In  Paramecium  the  reflex  type  of  behavior 
is  relatively  complete;  there  is  no  associative 
memory  and  no  ability  to  learn  by  experience. 
In  the  earthworm  associative  memory  is  but 
slightly  developed  and  the  animal  learns  but 
little  by  experience  and  can  make  no  applica- 
tion of  past  experiences  to  new  conditions.  In 
the  dog  associative  memory  is  well  developed; 
the  animal  learns  by  experience  and  can,  to  a 
limited  extent,  apply  such  memory  of  past  ex- 
periences to  new  conditions.  In  adult  man  all 
of  these  processes  are  fully  developed  and  par- 
ticularly the  last,  viz.,  the  ability  to  reason. 
But  in  his  development  the  human  individual 
passes  through  the  more  primitive  stages  of 
intelligence,  represented  by  the  lower  animals 
named;  the  germ  cells  and  embryo  represent 
only  the  stages  of  reflex  behavior,  to  these  trial 
and  error  and  associative  memory  are  added  in 
the  infant  and  young  child,  and  to  these  the 


FACTS  AND  FACTORS  OF  DEVELOPMENT        67 

application  of  past  experience  to  new  condi- 
tions, or  reason,  is  added  in  later  years. 

5.  Will. — Another  characteristic,  which 
many  persons  regard  as  the  supreme  psychical 
faculty,  is  the  will.  This  faculty  also  under- 
goes development  and  from  relatively  simple 
beginnings.  The  will  of  the  child  has  devel- 
oped out  of  something  which  is  far  less  perfect 
in  the  infant  and  embryo  than  in  the  child. 
Observations  and  experiments  on  lower  ani- 
mals and  on  human  beings,  as  well  as  intro- 
spective study  of  our  own  activities,  appear  to 
justify  the  following  conclusions: 

(1.)  Every  activity  of  an  organism  is  a 
response  to  one  or  more  stimuli,  external  or 
internal  in  origin.  These  stimuli  are  in  the 
main,  if  not  entirely,  energy  changes  outside 
or  inside  the  organism.  In  lower  organisms 
as  well  as  in  the  germ  cells  and  embryos  of 
higher  animals  the  possible  number  of  re- 
sponses are  few  and  prescribed  owing  to  their 
relative  simplicity,  and  the  response  follows 
the  stimulus  directly.  In  more  complex  or- 
ganisms the  number  of  possible  responses  to 
a  stimulus  is  greatly  increased,  and  the  visible 


68  HEREDITY  AND  ENVIRONMENT 

response  may  be  the  end  of  a  long  series  of 
internal  changes  which  are  started  by  the 
original  stimulus. 

(2.)  The  response  to  a  stimulus  may  be 
modified  or  inhibited  in  the  following  ways: 

(a)  Through  conflicting  stimuli  and 
changed  physiological  states,  due  to  fatigue, 
hunger,  etc.  Many  stimuli  may  reach  the 
organism  at  the  same  time  and  if  they  conflict 
they  may  nullify  one  another  or  the  organism 
may  respond  to  the  strongest  stimulus  and 
disregard  the  weaker  ones.  When  an  organ- 
ism has  begun  to  respond  to  one  stimulus  it  is 
not  easily  diverted  to  another.  Jennings  found 
that  the  attached  infusorian,  Stentor,  which 
usually  responds  to  strong  stimuli  by  closing 
up,  may,  when  repeatedly  stimulated,  loosen 
its  attachment  and  swim  away,  thus  respond- 
ing in  a  wholly  new  manner  when  its  physi- 
ological state  has  been  changed  by  repeated 
stimuli  and  responses.  Whitman  found  that 
leeches  of  the  genus  Clepsine  prefer  shade  to 
bright  light,  and  other  things  being  equal  they 
always  seek  the  under  sides  of  stones  and 
shaded  places ;  but  if  a  turtle  from  which  they 


FACTS  AND  FACTORS  OF  DEVELOPMENT        69 

normally  suck  blood  is  put  into  an  aquarium 
with  the  leeches,  they  at  once  leave  the  shade 
and  attach  themselves  to  the  turtle.  They 
prefer  shade  to  bright  light  but  they  prefer 
their  food  to  the  shade.  The  tendency  to  re- 
main concealed  is  inhibited  by  the  stronger 
stimulus  of  hunger.  On  the  other  hand  he 
found  that  the  salamander,  Necturus,  is  so 
timid  that  it  will  not  take  food,  even  though 
starving,  until  by  gradual  stages  and  gentle 
treatment  its  timidity  can  be  overcome  to  a 
certain  extent.  Here  fear  is  at  first  a  stronger 
stimulus  than  hunger  and  unless  the  stimulus 
of  fear  can  be  reduced  the  animal  will  starve 
to  death  in  the  presence  of  the  most  tempting 
food. 

(b)  Responses  may  also  be  modified 
through  compulsory  limitation  of  many  pos- 
sible responses  to  a  particular  one,  and  the  con- 
sequent formation  of  a  habit.  This  is  the 
method  of  education  employed  in  training  all 
sorts  of  animals.  Thus  Jennings  found  that 
a  star-fish  could  be  trained  to  turn  itself  over, 
when  placed  on  its  back,  by  means  of  one  par- 
ticular arm  simply  by  persistently  preventing 


70  HEREDITY  AND  ENVIRONMENT 

the  use  of  the  other  arms.  Many  responses  of 
organisms  are  modified  in  a  similar  way,  not 
only  by  artificial  limitations  but  also  by 
natural  ones. 

(c)  Responses  which  have  become  fixed  and 
constant  through  natural  selection  or  other 
means  of  limitation  may  become  more  varied 
and  general  when  the  compulsory  limitation  is 
relaxed.  Behavior  in  the  former  case  is  fixed 
and  instinctive,  in  the  latter  more  varied  and 
plastic.  Thus  Whitman  found  that  the  be- 
havior of  domesticated  pigeons  is  more  vari- 
able and  their  instincts  less  rigidly  fixed  than 
in  wild  species.  If  the  eggs  are  removed  to  a 
little  distance  from  the  nest  the  wild  passenger 
pigeon  returns  to  the  nest  and  sits  down  as 
if  nothing  had  happened.  She  soon  finds  out, 
not  by  sight  but  by  feeling,  that  something  is 
missing,  and  she  leaves  the  nest  after  a  few 
minutes  without  heeding  the  eggs.  The  ring- 
neck  pigeon  also  misses  the  eggs  and  some- 
times rolls  one  of  them  back  into  the  nest,  but 
never  attempts  to  recover  more  than  one.  The 
dove-cote  pigeon  generally  tries  to  recover 
both  eggs.  According  to  Whitman: 


FACTS  AND  FACTORS  OF  DEVELOPMENT        71 

In  these  three  grades  the  advance  is  from  extreme 
blind  uniformity  of  action,  with  little  or  no  choice,  to 
a  stage  of  less  rigid  uniformity.  .  .  .  Under  con- 
ditions of  domestication  the  action  of  natural  selec- 
tion has  been  relaxed,  with  the  result  that  the  rigor 
of  instinctive  co-ordination,  which  bars  alternative 
action,  is  more  or  less  reduced.  Not  only  is  the  door 
to  choice  thus  unlocked,  but  more  varied  opportuni- 
ties and  provocations  arise,  and  thus  the  internal 
mechanism  and  the  external  conditions  and  stimuli 
work  both  in  the  same  direction  to  favor  greater 
freedom  of  action.  When  choice  thus  enters  no  new 
factor  is  introduced.  There  is  greater  plasticity 
within  and  more  provocation  without,  and  hence  the 
same  bird,  without  the  addition  or  loss  of  a  single 
nerve  cell,  becomes  capable  of  higher  action  and  is 
encouraged  and  even  constrained  by  circumstances 
to  learn  to  use  its  privileges  of  choice.  Choice,  as  I 
conceive  it,  is  not  introduced  as  a  little  deity  en- 
capsuled  in  the  brain.  .  .  .  But  increased  plasticity 
invites  greater  interaction  of  stimuli  and  gives  more 
even  chances  for  conflicting  impulses. 

(d)  Finally  in  all  animals  behavior  is  modi- 
fied through  previous  experience,  just  as 
structure  is  also.  Where  several  responses  to  a 
stimulus  are  possible  and  where  experience  has 
taught  that  one  response  is  more  satisfactory 
than  another,  action  may  be  limited  to  this 
particular  response,  not  by  external  compul- 


72  HEREDITY  AND  ENVIRONMENT 

sion  but  by  the  internal  impulse  of  experience 
and  intelligence.  This  is  what  we  know  as 
conscious  choice  or  will.  Whitman  says: 

Choice  runs  on  blindly  at  first  and  ceases  to  be 
blind  only  in  proportion  as  the  animal  learns  through 
nature's  system  of  compulsory  education.  The  tele- 
ological  alternatives  are  organically  provided;  one  is 
taken  and  fails  to  give  satisfaction,  another  is  tried 
and  gives  contentment.  This  little  freedom  is  the 
dawning  grace  of  a  new  dispensation,  in  which  educa- 
tion by  experience  comes  in  as  an  amelioration  of  the 
law  of  elimination.  .  .  .  Intelligence  implies  varying 
degrees  of  freedom  of  choice,  but  never  complete 
emancipation  from  automatism. 

Freedom  of  action  does  not  mean  action 
without  stimuli,  but  rather  the  introduction  of 
the  results  of  experience  and  intelligence  as 
additional  stimuli.  The  activities  which  in 
lower  animals  are  "cabined,  cribbed,  confined," 
reach  in  man  their  fullest  and  freest  expres- 
sion; but  the  enormous  difference  between  the 
relatively  fixed  behavior  of  a  protozoan  or  a 
germ  cell  and  the  relatively  free  activities  of 
a  mature  man  is  bridged  not  only  in  the  proc- 
ess of  evolution,  but  also  in  the  course  of  indi- 
vidual development. 


FACTS  AND  FACTORS  OF  DEVELOPMENT        73 

6.  Consciousness. — The  most  complex  of 
all  psychic  phenomena,  indeed  the  one  which 
includes  many  if  not  all  of  the  others,  is  con- 
sciousness. Like  every  other  psychic  process 
this  has  undergone  development  in  each  of  us ; 
we  not  only  came  out  of  a  state  of  unconscious- 
ness, but  through  several  years  we  were  grad- 
ually acquiring  consciousness  by  a  process  of 
development.  Whether  consciousness  is  the 
sum  of  all  the  psychic  faculties,  or  is  a  new 
product  dependent  upon  the  interaction  of  the 
other  faculties,  it  must  pass  through  many 
stages  in  the  course  of  its  development,  stages 
which  would  commonly  be  counted  as  uncon- 
scious or  subconscious  states,  and  complete 
consciousness  must  depend  upon  the  complete 
development  and  activity  of  the  other  facul- 
ties, particularly  associative  memory  and  in- 
telligence. The  question  is  sometimes  asked 
whether  germ  cells,  and  indeed  all  living 
things,  may  not  be  conscious  in  some  vague 
manner.  One  might  as  well  ask  whether  water 
is  present  in  hydrogen  and  oxygen.  Doubtless 
the  elements  out  of  which  consciousness  de- 
velops are  present  in  the  germ  cells,  in  the 


74  HEREDITY  AND  ENVIRONMENT 

same  sense  that  the  elements  of  the  other 
psychic  processes  or  of  the  organs  of  the  body 
are  there  present;  not  as  a  miniature  of  the 
adult  condition,  but  rather  in  the  form  of  ele- 
ments or  factors,  which  by  a  long  series  of 
combinations  and  transformations,  due  to  in- 
teractions with  one  another  and  with  the  en- 
vironment, give  rise  to  the  fully  developed 
condition. 

Finally  there  seems  good  reason  for  believ- 
ing that  the  continuity  of  consciousness,  the 
continuing  sense  of  identity,  is  associated  with 
the  continuity  of  material  substance,  for  in 
spite  of  frequent  changes  of  the  materials  of 
which  we  are  composed  our  sense  of  identity 
remains  undisturbed.  However,  the  contin- 
uity of  protoplasmic  and  cellular  organization 
generally  remains  undisturbed  throughout  life, 
and  the  continuity  of  consciousness  is  asso- 
ciated with  this  continuity  of  organization, 
especially  in  certain  parts  of  the  brain.  It  is 
an  interesting  fact  that  in  man,  and  in  several 
other  animals  which  may  be  assumed  to  have 
a  sense  of  identity,  the  nerve  cells,  especially 
those  of  the  brain,  cease  dividing  at  an  early 


FACTS  AND  FACTORS  OF  DEVELOPMENT        75 

age,  and  these  identical  cells  persist  through- 
out the  remainder  of  life.  If  nerve  cells  con- 
tinued to  divide  throughout  life,  as  epithelial 
cells  do,  there  would  be  no  such  persistence 
of  identical  cells,  and  one  is  free  to  speculate 
that  in  such  cases  there  would  be  no  persis- 
tence of  the  sense  of  identity. 

Organization  includes  both,  structure  and 
function,  and  continuity  of  organization  im- 
plies not  only  persistence  of  protoplasmic  and 
cellular  structures  but  also  persistence  of  the 
functions  of  sensitivity,  reflexes,  memory,  in- 
stincts, intelligence,  and  will ;  the  continuity  of 
consciousness  is  associated  with  the  continuity 
of  these  activities,  as  well  as  with  the  struc- 
tures of  the  body  in  general  and  of  the  brain  in 
particular.  It  is  well  known  that  things  which 
interrupt  or  destroy  these  functions  or  struc- 
tures interrupt  or  destroy  consciousness. 
Lack  of  oxygen,  anesthetics,  normal  sleep 
cause  in  some  way  a  temporary  interruption 
of  these  functions  and  consequently  tempo- 
rary loss  of  consciousness;  while  certain  in- 
juries or  diseases  of  the  brain  which  bring 
about  the  destruction  of  certain  centers  or  as- 


76  HEREDITY  AND  ENVIRONMENT 

sociation  tracts  may  cause  permanent  loss  of 
consciousness. 

The  development  of  all  of  these  psychical 
faculties  runs  parallel  with  the  development 
of  bodily  structures  and  apparently  the 
method  of  development  in  the  two  cases  is 
similar,  viz.,  progressive  differentiation  of 
complex  and  specialized  structures  and  func- 
tions from  relatively  simple  and  generalized 
beginnings.  Indeed  the  entire  organism, 
structure  and  function,  body  and  mind,  is  a 
unity,  and  the  only  justification  for  dealing 
with  these  constituents  of  the  organism  as  if 
they  were  separate  entities,  whether  they  be 
regarded  in  their  adult  condition  or  in  the 
course  of  their  development,  is  to  be  found  in 
the  increased  convenience  and  effectiveness  of 
such  separate  treatment. 

Development,  like  many  other  vital  phe- 
nomena, may  be  considered  from  several  dif- 
ferent points  of  view,  such  as  (1)  physico- 
chemical  events  involved,  (2)  physiological 
processes,  (3)  morphological  characters,  (4) 
ecological  correlations  and  adaptations,  (5) 
psychological  phenomena,  (6)  social  and 


FACTS  AND  FACTORS  OF  DEVELOPMENT        77 

moral  developments.  All  of  these  phases  of 
development  are  correlated,  indeed  they  are 
parts  of  one  general  process,  and  a  complete 
account  of  this  process  must  include  them  all. 
General  considerations  may  lead  us  to  the  be- 
lief that  each  of  the  succeeding  aspects  of  de- 
velopment named  above  may  be  causally  ex- 
plained in  terms  of  the  preceding  ones,  and 
hence  all  be  reducible  to  physics  and  chemistry. 
But  this  is  not  now  demonstrable  and  may 
not  be  true.  Function  and  structure  may  be 
related  causally,  or  they  may  be  two  aspects 
of  one  substance.  The  same  is  true  of  body 
and  mind  or  of  matter  and  energy.  But  even 
if  each  of  these  different  phases  in  the  develop- 
ment of  personality  may  not  be  causally  ex- 
plained by  the  preceding  ones,  at  least  the 
principle  of  explanation  employed  for  any 
aspect  of  development  ought  to  be  consistent 
and  harmonious  with  that  employed  for  any 
other  aspect. 

The  phenomena  of  mental  development  in 
man  and  other  animals  may  be  summarized  as 
follows : 


78 


HEREDITY    AND    ENVIRONMENT 


DEVELOPMENT  OF  PSYCHICAL  PROCESSES  IN  ONTO- 
GENY AND  PHYLOGENY 


ALL  LIVING  THINGS,  IN- 
CLUDING GERM  CELLS  AND 
EMBRYOS,  SHOW: 

1.  Differential  Sensitivity  = 
Different     Responses     to 

Stimuli  differing  in  Kind 
or  Quantity. 

2.  Reflex  Motions  — 
Relatively   Simple,   Auto- 
matic  Responses. 

3.  Organic  Memory  = 
Results   of   Previous   Ex- 
perience    registered     in 
General  Protoplasm. 

4.  Adaptive    Responses  = 
Results  of  Elimination  of 

Useless    Responses 
through  Trial  and  Error. 

5.  Varied  Responses 
Dependent  upon  Conflict- 
ing  Stimuli   and    Physi- 
ological States. 

6.  Identity  =: 
Continuity    of    Individual 

Organization. 


MATURE    FORMS    OF    HIGHER 
ANIMALS  SHOW: 

1.  Special   Senses   and   Sen- 

sations — 

Sensations    are    the    Ele- 
.  ments  of  Mind. 

2.  Instincts  (Inherited), 

Habits    (Acquired)  = 
Complex  Reflexes,  involv- 
ing Nerve  Centers. 

3.  Associative  Memory  — 
Results      of      Experience 

registered  in  Nerve  Cen- 
ters and  Association 
Tracts. 

4.  Intelligence,    Reason  •=. 
Results  of  Trial  and  Er- 
ror     plus       Associative 
Memory,    i.    e.    Experi- 
ence. 

5.  Inhibition,  Choice,   Will 
Dependent  upon  Associa- 
tive     Memory,      Intelli- 
gence,   Reason. 

6.  Consciousness  = 
Continuity      of     Memory, 

Intelligence,  Reason, 
Will. 


FACTS  AND  FACTORS  OF  DEVELOPMENT        79 


B.  FACTORS  OF  DEVELOPMENT 

These  are  some  of  the  facts  of  development, 
— a  very  incomplete  resume  of  some  of  the 
stages  through  which  a  human  being  passes 
in  the  course  of  his  development  from  the 
germ.  What  are  the  factors  of  development? 
By  what  processes  is  it  possible  to  derive  from 
a  relatively  simple  germ  cell  the  complexities 
of  an  adult  animal?  How  can  mind  and  con- 
sciousness develop  out  of  the  relatively  simple 
psychical  elements  of  the  germ?  These  are 
some  of  the  great  problems  of  development — 
the  greatest  and  most  far-reaching  theme 
which  has  ever  occupied  the  minds  of  men. 

1.  Preformation. — When  the  mind  is  once 
lost  in  the  mystery  of  this  ever-recurring  mira- 
cle it  is  not  surprising  to  find  that  there  have 
been  those  who  have  refused  to  believe  it 
possible  and  who  have  practically  denied  de- 
velopment altogether.  The  old  doctrine  of 
"evolution,"  as  it  was  called  by  the  scientists  of 
the  eighteenth  century,  or  of  preformation  as 
we  know  it  to-day,  held  that  all  the  organs  or 


80  HEREDITY  AND  ENVIRONMENT 

parts  of  the  adult  were  present  in  the  germ  in 
a  minute  and  transparent  condition  as  the 
leaves  and  stem  are  present  in  a  bud,  or  as  the 
shoot  and  root  of  the  little  plant  are  present 
in  the  seed.2  In  the  case  of  animals  it  was 
generally  impossible  to  see  the  parts  of  the 
future  animal  in  the  germ,  but  this  was  sup- 
posed to  be  due  to  the  smaller  size  of  the  parts 
and  to  their  greater  transparency,  and  with 
poor  microscopes  and  good  imagination  some 
observers  thought  they  could  see  the  little  ani- 
mals in  the  egg  or  sperm,  and  even  the  little 
man,  or  "homunculus,"  was  described  and 
figured  as  folded  up  in  one  or  the  other  of  the 
sex  cells. 

This  doctrine  of  preformation  was  not  only 
an  attempt  to  solve  the  mystery  of  develop- 
ment, but  it  was  also  an  attempt  to  avoid  the 
theological  difficulties  supposed  to  be  involved 
in  the  view  that  individuals  are  produced  by  a 
process  of  gradual  development  rather  than 

'The  little  plant  in  the  seed  is  itself  the  product  of  the  de- 
velopment of  a  single  cell,  the  ovule,  in  which  no  trace  of  a 
plant  is  present,  but  of  course  this  fact  was  not  known  until 
after  careful  microscopical  studies  had  been  made  of  the 
earliest  stages  of  development. 


FACTS  AND  FACTORS  OF  DEVELOPMENT        81 

by  supernatural  creation.  If  every  individual 
of  the  race  existed  within  the  germ  cells  of  the 
first  parents,  then  in  the  creation  of  the  first 
parents  the  entire  race  with  its  millions  of  in- 
dividuals was  created  at  once.  Thus  arose  the 
theory  of  "emboitement,"  or  infinite  encase- 
ment, the  absurdities  of  which  contributed  to 
the  downfall  of  the  entire  doctrine  of  prefor- 
mation,  which,  in  the  form  given  it  by  many 
naturalists  of  the  eighteenth  century,  is  now 
only  a  curiosity  of  biological  literature. 

2.  Epigenesis. — As  opposed  to  this  doctrine 
of  preformation,  which  was  founded  largely 
on  speculation,  arose  the  theory  of  epigenesis, 
which  was  in  its  main  features  founded  upon 
the  direct  observation  of  development,  and 
which  maintained  that  the  germ  contains  none 
of  the  adult  parts,  but  that  it  is  absolutely 
simple  and  undifferentiated,  and  that  from 
these  simple  beginnings  the  individual  grad- 
ually becomes  complex  by  a  process  of  differ- 
entiation. We  owe  the  theory  of  epigenesis, 
at  least  so  far  as  its  main  features  are  con- 
cerned, to  William  Harvey,  the  discoverer  of 
the  circulation  of  the  blood,  and  to  Caspar 


82  HEREDITY  AND  ENVIRONMENT 

Friederich  Wolff,  whose  doctoral  thesis,  pub- 
lished in  1759  and  entitled  "Tlieoria  Genera- 
tionis"  marked  the  beginning  of  a  great  epoch 
in  the  study  of  development.  Wolff  demon- 
strated that  adult  parts  are  not  present  in  the 
germ,  either  in  animals  or  in  plants,  but  that 
these  parts  gradually  appear  in  the  process  of 
development.  He  held,  erroneously,  that  the 
germ  is  absolutely  simple,  homogeneous  and 
undifferentiated,  and  that  differentiation  and 
organization  gradually  appear  in  this  undiffer- 
entiated substance.  How  to  get  differentia- 
tions out  of  non-differentiated  material, 
heterogeneity  out  of  homogeneity,  was  the 
great  problem  which  confronted  Wolff  and  his 
followers,  and  they  were  compelled  to  assume 
some  extrinsic  or  environmental  force,  some 
vis  formativia  or  spiritus  rector,  which  could 
set  in  motion  and  direct  the  process  of 
development. 

The  doctrine  of  preformation,  by  locating 
in  the  germ  all  the  parts  which  would  ever 
arise  from  it,  practically  denied  development 
altogether;  epigenesis  recognized  the  fact  of 
development,  but  attributed  it  to  mysterious 


FACTS  AND  FACTORS  OF  DEVELOPMENT        83 

and  purely  hypothetical  external  forces;  the 
one  placed  all  emphasis  upon  the  germ  and  its 
structures,  the  other  upon  outside  forces  and 
conditions. 

3.  Preformation  and  Epigenesis. — Modern 
students  of  development  recognize  that  neither 
of  these  extreme  views  is  true — adult  parts 
are  not  present  in  the  germ,  nor  is  the  latter 
homogeneous — but  there  are  in  germ  cells 
many  different  structures  and  functions  which 
are,  however,  very  unlike  those  of  the  adult, 
and  by  the  transformation  and  differentiation 
of  this  germinal  organization  the  complicated 
organization  of  the  adult  arises.  Development 
is  not  the  unfolding  of  an  infolded  organism, 
nor  the  mere  sorting  of  materials  already  pres- 
ent in  the  germ  cells,  though  this  does  take 
place,  but  rather  it  consists  in  the  formation 
of  new  materials  and  qualities,  of  new  struc- 
tures and  functions — by  the  combination  and 
interaction  of  the  germinal  elements  present  in 
the  oosperm.  In  similar  manner  the  combina- 
tion and  interaction  of  chemical  elements  yield 
new  substances  and  qualities  which  are  not  to 
be  observed  in  the  elements  themselves.  Such 


84  HEREDITY  AND  ENVIRONMENT 

new  substances  and  qualities,  whether  in  the 
organic  or  in  the  inorganic  world,  do  not  arise 
by  the  gradual  unfolding  of  what  was  present 
from  the  beginning,  but  they  are  produced  by 
a  process  of  "creative  synthesis." 

Modern  studies  of  germ  cells  have  shown 
that  they  are  much  more  complex  than  was 
formerly  believed  to  be  the  case;  they  may 
even  contain  different  "organ-forming  sub- 
stances" which  in  the  course  of  development 
give  rise  to  particular  organs ;  these  substances 
may  be  so  placed  in  the  egg  as  to  foreshadow 
the  polarity,  symmetry  and  pattern  of  the 
embryo,  but  even  the  most  highly  organized 
egg  is  relatively  simple  as  compared  with  the 
animal  into  which  it  ultimately  develops.  In- 
creasing complexity,  which  is  the  essence  of 
development,  is  caused  by  the  combination  and 
interaction  of  germinal  substances  under  the 
influence  of  the  environment.  The  organiza- 
tion of  the  oosperm  may  be  compared  to  the 
arrangement  of  tubes  and  flasks  in  a  compli- 
cated chemical  operation ;  they  stand  in  a  defi- 
nite relation  to  one  another  and  each  contains 
specific  substances.  The  final  result  of  the 


FACTS  AND  FACTORS  OF  DEVELOPMENT        85 

operation  depends  not  merely  upon  the  sub- 
stances used,  nor  merely  upon  the  way  in  which 
the  apparatus  is  set  up,  but  upon  both  of  these 
things,  as  well  as  upon  the  environmental  con- 
ditions represented  by  temperature,  pressure, 
moisture  or  other  extrinsic  factors. 

4.  Heredity  and  Environment. — Unques- 
tionably the  factors  or  causes  of  development 
are  to  be  found  not  merely  in  the  germ  but 
also  in  the  environment,  not  only  in  intrinsic 
but  also  in  extrinsic  forces;  but  it  is  equally 
certain  that  the  directing  and  guiding  factors 
of  development  are  in  the  main  intrinsic,  and 
are  present  in  the  organization  of  the  germ 
cells,  while  the  environmental  factors  exercise 
chiefly  a  stimulating,  inhibiting  or  modifying 
influence  on  development.  In  the  same  dish 
and  under  similar  environmental  conditions, 
one  egg  will  develop  into  a  worm,  another  into 
a  sea  urchin,  another  into  a  fish,  and  it  is  cer- 
tain that  the  different  fate  of  each  egg  is  de- 
termined by  conditions  intrinsic  in  the  egg 
itself,  rather  than  by  environmental  conditions. 
We  should  look  upon  the  germ  as  a  living 
thing,  and  upon  development  as  one  of  its 


86  HEREDITY  AND  ENVIRONMENT 

functions.  Just  as  the  character  of  any  func- 
tion is  determined  by  the  organism,  though  it 
may  be  modified  by  environment,  so  the  char- 
acter of  development  is  determined  by  hered- 
ity, t .  e.,  by  the  organization  of  the  germ  cells, 
though  the  course  and  results  of  develop- 
ment may  be  modified  by  .environmental 
conditions. 

SUMMARY 

In  conclusion,  we  have  briefly  reviewed  in 
this  lecture  the  well  known  fact  that  every  liv- 
ing thing  in  the  world  has  come  into  existence 
by  a  process  of  development;  that  the  entire 
human  personality,  mind  as  well  as  body,  has 
thus  arisen;  and  that  the  factors  of  develop- 
ment may  be  classified  as  intrinsic  in  the  or- 
ganization of  the  germ  cells,  and  extrinsic  as 
represented  in  environmental  forces  and  con- 
ditions. The  intrinsic  factors  are  those  which 
are  commonly  called  heredity,  and  they  direct 
and  guide  development  in  the  main;  the  ex- 
trinsic or  environmental  factors  furnish  the 
conditions  in  which  development  takes  place 
and  modify,  more  or  less,  its  course. 


CHAPTER  II 

THE  CELLULAR  BASIS  OF  HERED- 
ITY AND  DEVELOPMENT 


CHAPTER  II 

THE  CELLULAR  BASIS  OF  HEREDITY  AND 
DEVELOPMENT 

A.  INTRODUCTORY 

Heredity  is  to-day  the  central  problem  of 
biology.  This  problem  may  be  approached 
from  many  sides,  that  of  the  observer,  the 
statistician,  the  practical  breeder,  the  experi- 
menter, the  embryologist,  the  cytologist;  but 
these  different  aspects  of  the  subject  may  be 
reduced  to  three  general  methods  of  study, 
(1)  the  observational  and  statistical,  (2)  the 
experimental,  (3)  the  cytological  and  embry- 
ological.  Before  taking  up  these  different  as- 
pects of  heredity  it  is  important  that  we  should 
have  clear  definitions  of  the  terms  employed 
and  a  fairly  accurate  conception  of  the  pro- 
cesses involved. 

1.  Definitions. — Heredity  originally  meant 

89 


90  HEREDITY  AND  ENVIRONMENT 

heirship,  or  the  transmission  of  property  from 
parents  to  children,  and  in  the  field  of  biology 
it  has  been  defined  erroneously  as  "the  trans- 
mission of  qualities  or  characteristics,  mental 
or  physical,  from  parents  to  offspring"  (Cen- 
tury Dictionary).  The  colloquial  meaning  of 
the  word  has  led  to  much  confusion  in  biology, 
for  it  carries  with  it  the  idea  of  the  transmis- 
sion from  one  generation  to  the  next  of  owner- 
ship in  property.  A  son  may  inherit  a  house 
from  his  father  and  a  farm  from  his  mother, 
the  house  and  farm  remaining  the  same  though 
the  ownership  has  passed  from  parents  to  son. 
And  when  it  is  said  that  a  son  inherits  his 
stature  from  his  father  and  his  complexion 
from  his  mother,  the  stature  and  complexion 
are  usually  thought  of  only  in  their  developed 
condition,  while  the  great  fact  of  development 
is  temporarily  forgotten.  Of  course  there  are 
no  "qualities"  or  "characteristics"  which  are 
"transmitted"  as  such  from  one  generation  to 
the  next.  Such  terms  are  not  without  fault 
when  used  merely  as  figures  of  speech,  but 
when  interpreted  literally,  as  they  frequently 
are,  they  are  altogether  misleading;  they  are 


THE   CELLULAR  BASIS  91 

the  result  of  reasoning  about  names  rather 
than  facts,  of  getting  far  from  phenomena 
and  philosophizing  about  them.  The  compari- 
son of  heredity  to  the  transmission  of  property 
from  parents  to  children  has  produced  con- 
fusion in  the  scientific  as  well  as  in  the  popu- 
lar mind.  It  is  only  necessary  to  recall  the 
most  elementary  facts  about  development  to 
recognize  that  in  a  literal  sense  developed  char- 
acteristics of  parents  are  never  transmitted  to 
children. 

2.  The  Transmission  Hypothesis. — And 
yet  the  idea  that  the  characteristics  of  adult 
persons  are  transmitted  from  one  generation 
to  the  next  is  a  very  ancient  one  and  was  uni- 
versally held  until  the  most  recent  times.  Be- 
fore the  details  of  development  were  known 
it  was  natural  to  suppose,  as  Hippocrates  did, 
that  white-flowered  plants  gave  rise  to  white- 
flowered  seeds  and  that  blue-eyed  parents  pro- 
duced blue-eyed  germs,  without  attempting  to 
define  what  was  meant  by  white-flowered  seed 
or  blue-eyed  germs.  And  even  after  the  facts 
of  development  were  fairly  well  known  it  was 
generally  held  that  the  germ  cells  were  pro- 


92  HEREDITY  AND  ENVIRONMENT 

duced  by  the  adult  animal  or  plant  and  that 
the  characteristics  of  the  adult  were  in  some 
way  carried  over  to  the  germ  cells;  but  the 
manner  in  which  this  supposed  transmission 
took  place  remained  undefined  until  Darwin 
attempted  to  explain  it  by  his  "provisional 
hypothesis  of  pangenesis."  Darwin  assumed 
that  minute  particles  or  "gemmules"  were 
given  off  by  every  cell  of  the  body,  at  every 
stage  of  development,  and  that  these  gem- 
mules  then  collected  in  the  germ  cells  which 
thus  became  storehouses  of  little  germs  from 
all  parts  of  the  body.  Afterward,  in  the  de- 
velopment of  the  germ  cells,  the  gemmules, 
or  little  germs,  developed  into  cells  and  organs 
similar  to  those  from  which  they  came. 

3.  Germinal  Continuity  and  Somatic  Dis- 
continuity.— Many  ingenious  hypotheses  have 
been  devised  to  explain  beliefs  which  are  not 
correct,  and  this  is  one  of  them.  The  doctrine 
that  adult  organisms  manufacture  germ  cells 
and  transmit  their  characters  to  them  is 
known  to  be  erroneous.  Neither  germ  cells 
nor  any  other  kind  of  cells  are  formed  by  the 
body  as  a  whole,  but  every  cell  in  the  body 


THE  CELLULAR  BASIS  93 

comes  from  a  preceding  cell  by  a  process  of  di- 
vision, and  germ  cells  are  formed,  not  by  con- 
tributions from  all  parts  of  the  body,  but  by 
division  of  preceding  cells  which  are  derived 
ultimately  from  the  fertilized  egg  (Fig.  22). 
The  hen  does  not  produce  the  egg,  but  the  egg 
produces  the  hen  and  also  other  eggs.  Indi- 
vidual traits  are  not  transmitted  from  the  hen 
to  the  egg,  but  they  develop  out  of  germinal 
factors  which  are  carried  along  from  cell  to 
cell,  and  from  generation  to  generation. 

There  is  a  continuity  of  germinal  substance, 
and  usually  of  germinal  cells,  from  one  gener- 
ation to  the  next.  In  some  animals  the  germ 
cells  are  set  apart  at  a  very  early  stage  of  de- 
velopment, sometimes  in  the  early  cleavage 
stages  of  the  egg.  In  other  cases  the  germ 
cells  are  first  recognizable  at  later  stages,  but 
in  practically  every  case  they  arise  from 
germinal  or  embryonic  cells  which  have  not 
differentiated  into  somatic  tissues.  Germinal 
continuity  and  somatic  discontinuity  of  suc- 
cessive generations  in  sexually  produced  or- 
ganisms is  not  a  theory  but  an  established  fact. 
In  general,  germ  cells  do  not  come  from  dif- 


94  HEREDITY  AND  ENVIRONMENT 

ferentiated  somatic  cells,  but  only  from  un- 
differentiated  germinal  cells,  and  if  in  a  few 
doubtful  cases  differentiated  cells  mav  reverse 


FIG.  22.  DIAGRAM  SHOW- 
ING THE  "CELL  LINEAGE"  OF 
THE  BODY  CELLS  AND  GERM 
CELLS  IN  A  WORM  OR  MOL- 
LUSK.  The  lineage  of  the 
germ  cells  ("germ  track") 
is  shown  in  black,  of  ecto- 
derm in  white,  and  of  endo- 
derm  and  mesoderm  in 
shaded  circles.  The  whole 
course  of  spermatogenesis 
and  oogenesis  is  shown  in 
the  lower  right  of  the  figure 
beginning  with  the  primitive 
sex  cells  (Prim.  Sex  Cells) 
and  ending  with  the  game- 
tes, the  genesis  of  the  sper- 
matozoa being  shown  on  the 
left  and  that  of  the  ova  on 
the  right. 


THE   CELLULAR   BASIS  95 

the  process  of  development  and  become  embry- 
onic cells  and  even  germ  cells  it  does  not 
destroy  this  general  principle  of  germinal  con- 
tinuity and  somatic  discontinuity. 

Thus  the  problem  which  faces  the  student 
of  heredity  and  development  has  been  cut  in 
two;  he  no  longer  inquires  how  the  body  pro- 
duces the  germ  cells,  for  this  does  not  happen, 
but  merely  how  the  latter  produce  the  body 
and  other  germ  cells.  The  germ  is  the  unde- 
veloped organism  which  forms  the  bond  be- 
tween successive  generations ;  the  person  is  the 
developed  organism  which  arises  from  the 
germ  under  the  influence  of  environmental 
conditions.  The  person  develops  and  dies  in 
each  generation ;  the  germ-plasm  is  the  contin- 
uous stream  of  living  substance  which  con- 
nects all  generations.  The  person  nourishes 
and  protects  the  germ,  and  in  this  sense  the 
person  is  merely  the  carrier  of  the  germ-plasm, 
the  mortal  trustee  of  an  immortal  substance. 

This  contrast  of  the  germ  and  the  person, 
of  the  undeveloped  and  the  developed  organ- 
ism, is  fundamental  in  all  modern  studies  of 
heredity.  It  was  especially  emphasized  by 


96  HEREDITY  AND  ENVIRONMENT 

Weismann  in  his  germ-plasm  theory  and 
recently  it  has  been  made  prominent  by 
Johannsen  under  the  terms  "genotype"  and 
"phenotype" ;  the  genotype  is  the  fundamental 
hereditary  constitution  of  an  organism,  it  is 
the  germinal  type;  the  phenotype  is  the  de- 
veloped organism  with  all  of  its  visible  char- 
acters,, it  is  the  somatic  type. 

But  important  as  this  distinction  is  between 
germ  and  soma  it  has  sometimes  been  over- 
emphasized. This  is  one  of  the  chief  faults  of 
Weismann's  theory.  The  germ  and  the  soma 
are  generically  alike,  but  specifically  different. 
Both  germ  cells  and  somatic  cells  have  come 
from  the  same  oosperm,  but  have  differentiated 
in  different  ways;  the  tissue  cells  have  lost 
certain  things  which  the  germ  cells  retain  and 
have  developed  other  things  which  remain  un- 
developed in  the  germ  cells.  But  the  germ 
cells  do  not  remain  undifferentiated ;  both  egg 
and  sperm  are  differentiated,  the  former  for 
receiving  the  sperm  and  for  the  nourishment 
of  the  embryo,  the  latter  for  locomotion  and 
for  penetration  into  the  egg.  But  while  the 
differentiations  of  tissue  cells  are  usually  irre- 


THE   CELLULAR  BASIS  97 

versible,  so  that  they  do  not  again  become 
germinal  cells,  the  differentiations  of  the  sex 
cells  are  reversible,  so  that  these  cells,  after 
their  union,  again  become  germinal  cells. 

In  many  theories  of  heredity  it  is  assumed 
that  there  is  a  specific  "inheritance  material," 
distinct  from  the  general  protoplasm,  of  which 
the  function  is  the  "transmission"  of  hereditary 
properties  from  generation  to  generation,  and 
of  which  the  characteristics  are  independence 
of  the  general  protoplasm,  continuity  from 
generation  to  generation  and  extreme  stability 
in  organization.  This  is  the  idioplasm  of 
Xageli,  the  germ-plasm  of  Weismann.  But 
there  is  no  reason  to  suppose  that  "germ- 
plasm"  is  anything  other  than  germinal  proto- 
plasm, which  is  found  in  all  cells  in  the  earliest 
stages  of  development  but  which  becomes 
limited  in  quantity  or  altered  in  quality  in 
tissue  cells.  A  "germ-plasm"  which  is  abso- 
lutely distinct  from  and  independent  of  the 
general  protoplasm  is  a  mere  fiction  which 
finds  no  justification  in  reality. 

4.  The  Units  of  Living  Matter.— The  en- 
tire cell,  nucleus  and  cytoplasm,  is  the  ulti- 


98  HEREDITY  AND  ENVIRONMENT 

mate  unit  of  living  matter  which  is  capable  of 
independent  existence.  Neither  the  nucleus 
nor  the  cytoplasm  can  for  long  live  independ- 
ently of  each  other,  but  the  entire  cell  can 
perform  all  the  fundamental  vital  processes. 
It  transforms  food  into  its  own  living  material, 
it  grows  and  divides,  it  is  capable  of  respond- 
ing to  many  kinds  of  stimuli.  But  while  the 
parts  of  a  cell  are  not  capable  of  independent 
existence  they  may  perform  certain  of  these 
vital  processes. 

Not  only  is  the  cell  as  a  whole  capable  of 
assimilation,  growth  and  division,  but  every 
living  part  of  the  cell  has  this  power.  The 
nucleus  builds  foreign  substances  into  its  own 
substance,  and  after  it  has  grown  to  a  certain 
size  it  divides  into  two ;  the  cytoplasm  does  the 
same,  and  this  process  of  assimilation,  growth 
and  division  occurs  in  many  parts  of  the  nu- 
cleus and  cytoplasm,  such  as  the  chromosomes, 
chromomeres,  centrosomes,  etc.  In  all  cases 
cells  come  from  cells,  nuclei  from  nuclei, 
chromosomes  from  chromosomes,  centrosomes 
from  centrosomes,  etc. 

Indeed,  the  manner  in  which  all  living  mat- 


THE   CELLULAR  BASIS  99 

ter  grows  indicates  that  every  minute  particle 
of  protoplasm  has  this  power  of  taking  in  food 
substance  and  of  dividing  into  two  particles 
when  it  has  grown  to  maximum  size.  Pre- 
sumably this  power  of  assimilation,  growth  and 
division  is  possessed  by  particles  of  protoplasm 
which  are  invisible  with  the  highest  powers  of 
our  microscopes,  though  it  is  probable  that 
these  particles  are  much  larger  than  the 
largest  molecules  known  to  chemistry.  The 
smallest  particle  which  can  be  seen  with  the 
most  powerful  microscope  in  ordinary  light  is 
about  250  (i(i  (millionths  of  a  millimeter)  in 
diameter.  The  largest  molecules  are  probably 
about  10  ftp  in  diameter.  Between  these  in- 
visible molecules  and  the  just  visible  particles 
of  protoplasm  there  may  be  other  units  of  or- 
ganization. These  hypothetical  particles  of 
protoplasm  have  been  supposed  by  many 
authors  to  be  the  ultimate  units  of  assimila- 
tion, growth  and  division,  and  in  so  far  as  these 
units  are  supposed  to  be  different  in  different 
species,  or  with  respect  to  different  hereditary 
characters,  they  are  known  as  inheritance  units. 
It  is  assumed  in  practically  all  theories  of 


100  HEREDITY  AND  ENVIRONMENT 

heredity  that  the  "inheritance  material,"  or 
more  correctly  the  germinal  protoplasm,  is 
composed  of  ultra-microscopical  units  which 
have  the  power  of  individual  growth  and  di- 
vision and  which  are  capable  of  undergoing 
many  combinations  and  dissociations  during 
the  course  of  development,  by  which  combina- 
tions and  dissociations  they  are  transformed 
into  the  structures  of  the  adult.  Various 
names  have  been  given  to  such  units  by  differ- 
ent authors;  they  are  the  "physiological  units" 
of  Herbert  Spencer,  the  "gemmules"  of  Dar- 
win, the  "plastidules"  of  Elsberg  and  Haeckel, 
the  "pangenes"  of  de  Vries,  the  "plasomes"  of 
Wiesner,  the  "idioblasts"  of  Hertwig,  the  "bio- 
phores"  and  "determinants"  of  Weismann. 

With  the  publication  of  Weismann's  work 
on  the  germ-plasm  in  1892  speculation  with 
regard  to  these  ultra-microscopic  units  of  life 
and  of  heredity  reached  a  climax  and  began 
to  decline,  owing  to  the  highly  speculative 
character  of  the  evidence  as  to  the  existence, 
nature  and  activities  of  such  units.  But  with 
the  rediscovery  of  Mendel's  principles  of 
heredity  the  necessity  of  assuming  the  exist- 


THE   CELLULAR  BASIS  101 

ence  of  inheritance  units  of  some  kind  once 
more  became  evident,  and,  without  attempting 
to  define  what  such  units  are  or  how  they  be- 
have modern  students  of  heredity  invariably 
accept  their  existence.  They  are  now  called 
determiners  or  factors  or  genes,  and  are  usu- 
ally thought  of  as  elements  or  units  of  the 
germ  cells  which  condition  the  characters  of 
the  developed  organism,  and  which  are  in  a 
measure  independent  of  one  another;  though 
of  course  neither  they  nor  any  other  parts  of 
a  cell  are  really  independent  in  the  sense  that 
they  can  exist  apart  from  one  another.  They 
are  to  be  thought  of  as  we  think  of  certain 
chemical  radicals  which  exist  only  in  combina- 
tion with  other  chemical  elements  in  the  form 
of  molecules,  and  yet  may  preserve  their  iden- 
tity in  many  different  combinations. 

If  there  are  inheritance  units,  such  as  de- 
terminers or  genes,  as  practically  all  students 
of  heredity  maintain,  they  must  be  contained 
in  the  germ  cells,  and  it  becomes  one  of  the 
fundamental  problems  of  biology  to  find  out 
where  and  what  these  units  are.  But  whether 
we  assume  the  existence  of  these  units  or  not 


102  HEREDITY  AND  ENVIRONMENT 

we  know  that  the  germ  cells  are  exceedingly 
complex,  that  they  contain  many  visible  units 
such  as  chromosomes,  chromomeres,  plasto- 
somes  and  microsomes,  and  that  with  every 
great  improvement  in  the  microscope  and  in 
microscopical  technique  other  structures  are 
made  visible  which  were  invisible  before,  and 
whether  the  particular  hypothetical  units  just 
named  are  present  or  not  seems  to  be  a  matter 
of  no  great  importance,  seeing  that,  so  far  as 
the  analysis  of  the  microscope  is  able  to  go, 
there  are  in  all  protoplasm  differentiated 
units  which  are  combined  into  a  system;  in 
short,  there  is  organization. 

5.  Heredity  and  Development. — The  germ 
cells  are  individual  entities  and  after  the  ferti- 
lization of  the  egg  the  new  individual  thus 
formed  remains  distinct  from  every  other  in- 
dividual. Furthermore,  from  its  earliest  to 
its  latest  stage  of  development  it  is  one  and 
the  same  organism;  the  egg  is  not  one  being 
and  the  embryo  another  and  the  adult  a  third, 
but  the  egg  of  a  human  being  is  a  human  being 
in  the  one-celled  stage  of  development,  and  the 
characteristics  of  the  adult  develop  out  of  the 


THE   CELLULAR  BASIS  103 

egg  and  are  not  in  some  mysterious  way 
grafted  upon  it  or  transmitted  to  it. 

Parents  do  not  transmit  their  characters  to 
their  offspring  but  their  germ  cells  in  the 
course  of  long  development  give  rise  to  adult 
characters  similar  to  those  of  the  parents.  The 
thing  which  persists  more  or  less  completely 
from  generation  to  generation  is  the  organiza- 
tion of  the  germ  cells  which  differentiate  in 
similar  ways  in  successive  generations  if  the  ex- 
trinsic factors  of  development  remain  similar. 

In  short,  heredity  may  be  defined  as  the 
appearance  in  offspring  of  characters  whose 
differential  causes  are  found  in  the  germ  cells. 
Heritage  is  the  sum  of  all  those  qualities  which 
are  determined  or  caused  by  this  germinal  or- 
ganization. Development  is  progressive  and 
coordinated  differentiation  of  this  germinal  or- 
ganization, by  which  it  is  transformed  into  the 
adult  organization.  Differentiation  is  the  for- 
mation and  localization  of  many  different  kinds 
of  substances  out  of  the  germinal  substance,  of 
many  different  structures  and  functions  out  of 
the  relatively  simple  structures  and  functions 
of  the  oosperm. 


104  HEREDITY  AND  ENVIRONMENT 

This  germinal  organization  influences  not 
merely  adult  characters  but  also  the  character 
of  every  stage  from  the  egg  to  the  adult  con- 
dition. For  every  inherited  character,  whether 
embryonic  or  adult,  there  is  some  germinal 
basis.  We  receive  from  our  parents  germ  cells 
of  a  particular  kind  and  constitution,  and 
under  given  conditions  of  environment  these 
cells  undergo  regular  transformations  and 
differentiations  in  the  course  of  development, 
differentiations  which  lead  to  particular  adult 
characteristics.  In  the  last  analysis  the  causes 
of  heredity  and  development  are  problems  of 
cell  structures  and  functions,  problems  of  the 
formation  of  particular  kinds  of  germ  cells,  of 
the  fusion  of  these  cells  in  fertilization,  and  of 
the  subsequent  formation  of  the  various  types 
of  somatic  cells  from  the  fertilized  egg  cell. 

B.  THE  GERM  CELLS 
Observations  and  experiments  on  developed 
animals  and  plants  have  furnished  us  with  a 
knowledge  of  the  finished '  products  of  inheri- 
tance, but  the  actual  stages  and  causes  of  in- 
heritance, the  real  mechanisms  of  heredity,  are 


THE   CELLULAR  BASIS  105 

to  be  found  only  in  a  study  of  the  germ  cells 
and  of  their  development.  Although  many 
phenomena  of  inheritance  have  been  dis- 
covered in  the  absence  of  any  definite  knowl- 
edge of  the  mechanism  of  heredity,  a  scientific 
explanation  of  these  phenomena  must  wait 
upon  the  knowledge  of  their  causes.  In  the 
absence  of  such  knowledge  it  has  been  neces- 
sary to  formulate  theories  of  heredity  to  ac- 
count for  the  facts,  but  these  theories  are  only 
temporary  scaffolding  to  bridge  the  gaps  in 
our  knowledge,  and  if  we  knew  all  that  could 
be  known  about  the  germ  cells  and  their  de- 
velopment we  should  have  little  need  of 
theories.  In  the  first  lecture  we  looked  at  the 
germ  cells  and  their  development  from  the 
outside,  as  it  were ;  let  us  now  look  inside  these 
cells  and  study  their  minuter  structures  and 
functions. 

Only  a  beginning  has  been  made  in  this 
minute  study  of  the  germ  cells  and  of  their 
transformation  into  the  developed  animal, 
and  it  seems  probable  that  it  may  engage  the 
attention  of  many  future  generations  of  bi- 
ologists, but  nevertheless  we  have  come  far 


FIG.   23 — For   Description   see   page   107 


THE  CELLULAR  BASIS  107 

since  that  day,  only  about  thirty-five  years 
ago,  when  Oscar  Hertwig  first  saw  the  ap- 
proach and  union  of  the  egg  and  sperm  nuclei 
within  the  fertilized  egg.  Indeed  so  rapid  has 
been  the  advance  of  knowledge  in  this  field 
that  many  of  the  pioneers  in  this  work  are 
still  active  in  research. 

1.  Fertilization. — The  development  of  the 
individual  may  be  said  to  begin  with  the  ferti- 
lization of  the  egg,  though  it  is  evident  that 
both  egg  and  sperm  must  have  had  a  more  re- 
mote beginning,  and  that  they  also  have  un- 
dergone a  process  of  development  by  which 
their  peculiar  characteristics  of  structure  and 
function  have  arisen;  a  subject  to  which  we 
shall  return  later.  But  the  developmental 
processes  which  lead  to  the  formation  of  fully 


FIG.  23.  DIAGRAMS  OF  THE  MATURATION  AND  FERTILIZATION 
OF  THE  EGG  OF  A  MOLLUSK  (Crepidula).  A,  B.  First  matura- 
tion division  (1st  Mat.  Sp.)  C.  Second  maturation  division 
(2d  Mat.  Sp.)  and  first  polar  body  (1st  PB)  resulting  from 
first  division.  <$N,  sperm  nucleus,  gC,  sperm  centrosome. 

D.  Approach   of   sperm   nucleus    (<£ZV)    and  sphere    (c?*S)    to 
egg  nucleus    ($JV)    and  sphere    ($/S) ;   the   second   polar  body 
(2d  PB)  has  been  formed  and  the  first  has  divided  (1st  PB). 

E.  Meeting  of  egg  and  sperm  nuclei  and  origin  of  cleavage 
centrosomes.     F.    First  cleavage  of  egg  showing  direction  of 
currents  in  the  cell. 


108  HEREDITY  AND  ENVIRONMENT 

developed  ova  and  spermatozoa  come  to  a  full 
stop  before  fertilization  and  they  do  not  usu- 
ally begin  again  until  a  spermatozoon  has 
entered  an  ovum,  or  until  the  latter  has  been 
stimulated  by  some  other  outside  means.  In 
some  animals  and  plants  eggs  may  develop 
regularly  without  fertilization,  the  stimulus  to 
development  being  supplied  by  certain  ex- 
ternal or  internal  conditions;  in  other  cases, 
as  Loeb  discovered,  eggs  which  would  never 
develop  if  left  to  themselves  may  be  experi- 
mentally stimulated  by  physical  or  chemical 
changes  in  the  environment,  so  that  they  un- 
dergo regular  development.  The  development 
of  an  egg  without  previous  fertilization  is 
known  as  parthenogenesis  or  virgin  reproduc- 
tion; if  it  occurs  in  nature  it  is  natural  par- 
thenogenesis, if  in  experiments  it  is  artificial 
parthenogenesis.  Natural  parthenogenesis  is 
relatively  rare  and  in  the  vast  majority  of 
animals  and  plants  the  egg  does  not  begin  to 
develop  until  a  spermatozoon  has  entered  it. 

But  the  spermatozoon  not  only  stimulates 
the  egg  to  develop,  as  environmental  condi- 
tions may  also  do,  but  it  carries  into  the  egg 


THE   CELLULAR  BASIS  109 

living  substances  which  are  of  great  signifi- 
cance in  heredity.  Usually  only  the  head  of 
the  spermatozoon  enters  the  egg  (Figs.  4-7) 
and  this  consists  almost  entirely  of  nuclear 
material  which  has  a  strong  chemical  affinity 
for  certain  dyes,  and  hence  is  called  chromatin 
(Fig.  23  A  and  B)  ;  when  the  egg  has  ma- 
tured and  is  ready  to  be  fertilized  its  nucleus 
also  consists  of  a  small  mass  of  chromatin 
(Fig.  23  C).  Both  of  these  condensed  chro- 
matic nuclei  then  grow  in  size  and  become  less 
chromatic  by  absorbing  from  the  egg  a  sub- 
stance which  is  not  easily  stained  by  dyes  and 
hence  is  called  achromatin  (Fig.  23  D  and  E) . 
The  chromatin  then  becomes  scattered  through 
each  nucleus  in  the  form  of  granules  or  threads 
which  are  embedded  in  the  achromatin;  this  is 
the  condition  of  a  typical  "resting"  nucleus. 
The  spermatozoon  also  brings  into  the  egg  a 
centrosome  or  division  center,  around  which 
an  aster  appears  consisting  of  radiating  lines 
in  the  protoplasm  of  the  egg  ( Fig.  23  B ) . 

The  moment  that  the  spermatozoon  touches 
the  surface  of  the  egg  the  latter  throws  out  at 
the  point  touched  a  prominence,  or  reception 


110  HEREDITY  AND  ENVIRONMENT 

cone  (Fig.  4),  and  as  soon  as  the  head  of  the 
sperm  has  entered  this  cone  some  of  the  super- 
ficial protoplasm  of  the  egg  flows  to  this  point 
and  then  turns  into  the  interior  of  the  egg  in 
a  kind  of  vortex  current.  Probably  as  a  re- 
sult of  this  current  the  sperm  nucleus  and 
centrosome  are  carried  deeper  into  the  egg  and 
finally  are  brought  near  to  the  egg  nucleus 
(Fig.  23  D  and  E).  In  the  movements  of 
egg  and  sperm  nuclei  toward  each  other  it  is 
evident  that  they  are  passively  carried  about 
by  currents  in  the  cytoplasm;  the  entrance  of 
the  sperm  serves  as  a  stimulus  to  the  egg  cyto- 
plasm which  moves  according  to  its  preestab- 
lished  organization. 

2.  Cleavage  and  Differentiation. — When 
the  sperm  nucleus  has  come  close  to  the  egg 
nucleus  the  sperm  centrosome  usually  divides 
into  two  minute  granules,  the  daughter  centro- 
somes,  which  move  apart  forming  a  spindle 
with  the  centrosomes  at  its  poles  and  with 
astral  radiations  running  out  from  these  into 
the  cytoplasm  (Fig.  23  F) .  At  the  same  time 
the  chromatin  granules  and  threads  in  the  egg 
and  sperm  nuclei  run  together  into  a  smooth 


THE   CELLULAR  BASIS  111 

thick  thread,  the  spireme,  which  is  coiled  within 
the  nucleus.  At  this  stage  it  is  sometimes  pos- 
sible to  see  that  the  spireme  is  composed  of  a 
series  of  granules,  like  beads  on  a  string;  these 
granules  are  the  chromomeres.  The  spireme 
then  breaks  up  into  a  number  of  pieces  in  the 
form  of  short  threads  or  rods  (Fig.  24  C  and 
D) ;  these  are  the  chromosomes.  The  number 
of  these  chromosomes  is  constant  for  every 
species  and  race,  though  the  number  may  vary 
in  different  species.  In  the  thread  worm, 
Ascaris  megalocephala,  there  are  usually  two 
chromosomes  in  the  egg  nucleus  and  two  in  the 
sperm  nucleus  (Fig.  24  D).  In  the  gastro- 
pod, Crepidula,  there  are  about  thirty  chromo- 
somes in  each  germ  nucleus  and  sixty  in  the 
two. 

Then  the  spindle  and  asters  grow  larger  and 
the  nuclear  membrane  grows  thinner  and 
finally  disappears  altogether,  leaving  the  chro- 
mosomes in  the  equator  of  the  spindle  (Figs. 
23  Ft  24  E  and  F,  25  I) .  Each  of  the  chromo- 
somes then  splits  lengthwise  into  two  equal 
parts,  and  in  the  splitting  of  the  chromosomes 


FIG.  2-t.  FERTILIZATION  OF  THE  EGG  OF  THE  NEJIATODE 
WORM  Ascaris  megalocephala;  JIV,  egg  nucleus;  £N,  sperm 
nucleus;  Arch,  archiplasm;  C,  centrosome;  A,  B,  approach  of 
germ  nuclei;  C,  D,  formation  of  two  chromosomes  in  each 
germ  nucleus;  E,  F,  stages  in  the  division  of  the  chromosomes 
which  are  split  in  E  and  are  separating  in  F;  only  three 
chromosomes  are  shown  in  F.  (From  Wilson  after  Boveri.) 


THE   CELLULAR   BASIS  113 

it  is  sometimes  possible  to  see  that  each  bead- 
like  chromomere  divides  through  its  middle. 
The  daughter  chromosomes  then  separate  and 
move  to  opposite  poles  of  the  spindle,  where 
they  form  the  daughter  nuclei,  and  at  the  same 
time  the  cell  body  begins  to  divide  by  a  con- 
striction which  pinches  the  cell  in  two  in  the 
plane  which  passes  through  the  equator  of  the 
spindle  (Figs.  24  F,  26  B) .  Finally  the 
daughter  nuclei  grow  in  size  by  the  absorp- 
tion of  achromatin  from  the  cell  body  and  the 
substance  of  the  chromosomes  is  again  scat- 
tered through  the  achromatin  in  the  form 
of  threads  and  granules  and  thus  the  daughter 
nuclei  come  back  to  a  "resting"  stage  similar 
to  that  with  which  the  division  began,  thus 
completing  the  "division  cycle"  of  the  cell. 

During  the  whole  division  cycle  it  is  possi- 
ble in  a  few  instances  to  distinguish  the  chro- 
mosomes of  the  egg  from  those  of  the  sperm, 
and  in  every  instance  where  this  can  be  done 
it  is  perfectly  clear  that  these  chromosomes  do 
not  fuse  together  nor  lose  their  identity,  but 
that  every  chromosome  splits  lengthwise  and 
its  halves  separate  and  go  into  the  two  daugh- 


114 


HEREDITY  AND  ENVIRONMENT 


FIG.  25.  MATURATION  AND  FERTILIZATION  OF  THE  EGG  OF 
THE  MOUSE.  A,  first  polar  body  and  second  maturation  spin- 
dle; B,  second  polar  body  and  maturation  spindle;  C,  entrance 
of  the  spermatozoon  into  the  egg;  D-G,  successive  stages  in 
the  approach  of  egg  and  sperm  nuclei;  //,  formation  of 
chromosomes  in  each  germ  nucleus;  /,  first  cleavage  spindle 
showing  chromosomes  from  egg  and  sperm  on  opposite  sides 
of  spindle.  (After  Sobotta.) 


THE   CELLULAR  BASIS  115 

ter  cells  where  they  form  the  daughter  nuclei. 
Each  of  these  cells  therefore  receives  half  of 
its  chromosomes  from  the  egg  and  half  from 
the  sperm.  Even  in  cases  where  the  individual 
chromosomes  are  lost  to  view  in  the  daughter 
nuclei  those  nuclei  may  sometimes  be  clearly 
double,  one-half  of  each  having  come  from  the 
egg  chromosomes  and  the  other  half  from  the 
sperm  chromosomes  (Fig.  26). 

At  every  subsequent  cleavage  of  the  egg 
the  chromosomes  divide  in  exactly  the  same 
way  as  has  been  described  for  the  first  cleav- 
age. Every  cell  of  the  developing  animal  re- 
ceives one-half  of  its  chromosomes  from  the 
egg  and  the  other  half  from  the  sperm,  and 
if  the  chromosomes  of  the  egg  differ  in  shape 
or  in  size  from  those  of  the  sperm,  as  is  some- 
times the  case  when  different  races  or  species 
are  crossed,  these  two  groups  of  chromosomes 
may  still  be  distinguished  at  advanced  stages 
of  development.  Where  the  egg  and  sperm 
chromosomes  are  not  thus  distinguishable  it 
may  still  be  possible  to  recognize  the  half  of 
the  nucleus  which  comes  from  the  egg  and  the 
half  which  comes  from  the  sperm  even  up  to 


116  HEREDITY  AND  ENVIRONMENT 


FIG.  26.  SUCCESSIVE  STAGES  IN  THE  CLEAVAGE  OF  THE  EGG 
OF  A  MOLLUSK  (Crepidula),  showing  the  separateness  of  the 
male  and  female  chromosomes  (£  ch,  $  ch)  and  of  the  male 
and  female  halves  of  each  nucleus  N,  N. 


THE  CELLULAR  BASIS  117 

an  advanced  stage  of  the  cleavage  (Fig.  26). 
At  the  same  time  that  the  maternal  and 
paternal  chromosomes  are  being  distributed 
with  such  precise  equality  to  all  the  cells  of 
the  developing  organism  the  different  sub- 
stances in  the  cell  body  outside  of  the  nucleus 
may  be  distributed  very  unequally  to  the  cleav- 
age cells.  The  movements  of  the  cytoplasm 
of  the  egg  which  began  with  the  flowing  of 
the  surface  layer  to  the  point  of  entrance  of 
the  sperm  lead  to  the  segregation  of  different 
kinds  of  plasms  in  different  parts  of  the  egg 
and  to  the  unequal  distribution  of  these  sub- 
stances to  different  cells. 

One  of  the  most  striking  cases  of  this  is 
found  in  the  ascidian,  Styela,  in  which  there 
are  four  or  five  different  kinds  of  substance  in 
the  egg  which  differ  in  color,  so  that  their  dis- 
tribution to  different  regions  of  the  egg  and 
to  different  cleavage  cells  may  be  easily  fol- 
lowed, and  even  photographed,  while  in  the 
living  condition.  The  peripheral  layer  of 
protoplasm  is  yellow  and  when  it  gathers  at 
the  lower  pole  of  the  eg|?  where  the  sperm 
enters  it  forms  a  yellow  cap  (Fig.  27,  1). 


118  HEREDITY  AND  ENVIRONMENT 

This  yellow  substance  then  moves,  following 
the  sperm  nucleus,  up  to  the  equator  of  the 
egg  on  the  posterior  side  and  there  forms  a 
yellow  crescent  extending  around  the  posterior 
side  of  the  egg  just  below  the  equator  (Fig. 
27,  3) .  On  the  anterior  side  of  the  egg  a  gray 
crescent  is  formed  in  a  somewhat  similar  man- 
ner and  at  the  lower  pole  between  these  two 
crescents  is  a  slate  blue  substance,  while  at  the 
upper  pole  is  an  area  of  colorless  protoplasm. 
The  yellow  crescent  goes  into  cleavage  cells 
which  become  muscle  and  mesoderm,  the  gray 
crescent  into  cells  which  become  nervous  sys- 
tem and  notochord,  the  slate  blue  substance 
into  endoderm  cells  and  the  colorless  substance 
into  ectoderm  cells. 

Thus  within  a  few  minutes  after  the  ferti- 
lization of  the  egg,  and  before  or  immediately 
after  the  first  cleavage,  the  anterior  and  pos- 
terior, dorsal  and  ventral,  right  and  left  poles 
are  clearly  distinguishable,  and  the  substances 
which  will  give  rise  to  ectoderm,  endoderm, 
mesoderm,  muscles,  notochord  and  nervous 
system  are  plainly  visible  in  their  character- 
istic positions. 


THE   CELLULAR   BASIS  119 

At  the  first  cleavage  of  the  egg  each  of  these 
substances  is  divided  into  right  and  left  halves 
( Fig.  27,  5 ) .  The  second  cleavage  «uts  off  two 
anterior  cells  containing  the  gray  crescent 
from  two  posterior  ones  containing  the  yellow 
crescent  (Fig.  27,  6  and  Fig.  28,  1).  The 
third  cleavage  separates  the  colorless  proto- 
plasm in  the  upper  hemisphere  from  the  slate 
blue  in  the  lower  (Fig.  28,  2) .  And  at  every 
successive  cleavage  the  cytoplasmic  substances 
are  segregated  and  isolated  in  particular  cells, 
and  in  this  way  the  cytoplasm  of  the  differ- 
ent cells  comes  to  be  unlike  (Figs.  28  and  29) . 
When  once  partition  walls  have  been  formed 
between  cells  they  permanently  separate  the 
substances  in  the  different  cells  so  that  they 
can  no  longer  commingle. 

What  is  true  of  Styela  in  this  regard  is 
equally  true  of  many  other  ascidians,  as  well 
as  of  Amphioscus  and  of  the  frog,  though  the 
segregation  of  substances  and  the  differentia- 
tion of  cells  are  not*  so  evident  in  the  last  named 
animals  because  these  substances  are  not  so 
strikingly  colored.  Indeed  the  segregation 
and  isolation  of  different  protoplasmic  sub- 


1P.S 


FIG.  27. 

FIG.  27.  SECTIONS  OF  THE  EGG  OF  Styela,  showing  matura- 
tion, fertilization  and  early  cleavage;  1  P.  8.,  first  polar 
spindle,  p.b.,  polar  bodies,  rfN,  sperm  nucleus,  $N,  egg 
nucleus,  p.l,  peripheral  layer  of  yellow  protoplasm,  Cr. 
crescent  of  yellow  protoplasm,  Ap  Av  anterior  cells,  B3, 
jBj,  posterior  cells  of  the  4-cell  stage.  In  1  the  sperm  nucleus 
and  centrosome  are  at  the  lower  pole  near  the  point  of 


FIG.  28. 

entrance;  in  2  and  3  they  have  moved  up  to  the  equator  on 
the  posterior  side  of  the  egg;  in  4  the  egg  and  sperm  nuclei 
have  come  together  and  the  sperm  centrosome  has  divided  and 
formed  the  cleavage  spindle;  in  5  the  egg  is  dividing  into 
right  and  left  halves;  in  6  it  is  dividing  into  anterior  and 
posterior  halves. 

FIG.  28.  CLEAVAGE  or  THE  EGG  OF  Styela,  showing  distribu- 
tion of  the  yellow  protoplasm  (stippled)  and  of  the  clear  and 
gray  protoplasm  to  the  various  cells,  each  of  which  bears  a 
definite  letter  and  number. 


122  HEREDITY  AND  ENVIRONMENT 

stances  in  different  cleavage  cells  occurs  dur- 
ing the  cleavage  of  the  egg  in  all  animals, 
though  such  differentiations  are  much  more 
marked  in  some  cases  than  in  others. 

This  same  type  of  cell  division,  with  equal 
division  of  the  chromosomes  and  more  or  less 
unequal  division  of  the  cell  body,  continues 
long  after  the  cleavage  stages,  indeed 
throughout  the  entire  period  of  embryonic 
development.  Sometimes  the  division  of  the 
cell  body  is  equal,  the  daughter  cells  being 
alike;  sometimes  it  is  unequal  or  differential, 
but  always  the  division  of  the  chromosomes  is 
equal  and  non-differential.  When  once  the 
various  tissues  have  been  differentiated  the 
further  divisions  in  these  tissue  cells  are  usu- 
ally non-differential  even  in  the  case  of  the 
cell  bodies. 

There  can  be  no  doubt  that  this  remarkably 
complicated  process  of  cell  division  has  some 
deep  significance ;  why  should  a  nucleus  divide 
in  this  peculiarly  indirect  manner  instead  of 
merely  pinching  in  two,  as  was  once  supposed 
to  be  the  rule?  What  is  the  relation  of  cell 
division  to  embryonic  differentiation?  In  this 


THE   CELLULAR   BASIS  123 

'process  of  mitosis,  or  indirect  cell  division,  two 
important  things  take  place:  (1)  Each 
chromosome,  chromomere  and  centrosome  is 
divided  exactly  into  two  equal  parts  so  that 
each  daughter  structure  is  at  the  time  of  its 
formation  quantitatively  one-half  the  size  and 
qualitatively  precisely  like  its  mother  struc- 
ture. (2)  Accompanying  the  formation  of 
radiations,  which  go  out  from  the  centrosomes 
into  the  cell  body,  diffusion  currents  are  set 
up  in  the  cytoplasm  which  lead  to  the  localiza- 
tion of  different  parts  of  the  cytoplasm  in 
definite  regions  of  the  cell,  and  this  cytoplas- 
mic  localization  is  sometimes  of  such  a  sort 
that  one  of  the  daughter  cells  may  contain  one 
kind  of  cell  substance  and  the  other  another 
kind.  Thus  while  mitosis  brings  about  -a 
scrupulously  equal  division  of  the  elements  of 
the  nucleus,  it  may  lead  to  a  very  unequal  and 
dissimilar  division  of  the  cytoplasm.  In  this 
is  found  the  significance  of  mitosis,  and  it  sug- 
gests at  once  that  the  nucleus  contains  undif- 
ferentiating  material,  viz.,  the  idioplasm  or 
germ-plasm,  which  is  characteristic  of  the  race 
and  is  carried  on  from  cell  to  cell  and  from 


FIG.  29.  GASTRUXA  AND  LARVA  OF  Styela,  showing  the  cell 
lineage  of  various  organs,  and  the  distribution  of  the  differ- 
ent kinds  of  protoplasm  to  these  organs.  Muscle  cells  are 
shaded  by  vertical  lines,  mesenchyme  by  horizontal  lines,  ner- 
vous system  and  chorda  by  stipples. 


THE   CELLULAR   BASIS  135 

generation  to  generation;  whereas  the  cell 
body  contains  the  differentiating  substance, 
the  personal  plasm  or  somatoplasm  which 
gives  rise  to  all  the  differentiations  of  cells, 
tissues  and  organs  in  the  course  of  ontogeny. 

Weismann  supposed  that  the  mitotic  divi- 
sion of  the  chromosomes  during  development 
was  of  a  differential  character,  the  daughter 
chromosomes  differing  from  each  other  at 
every  differential  division  in  some  constant 
and  characteristic  way,  and  that  these  differ- 
entiations of  the  chromosomes  produced  the 
characteristic  differentiations  of  the  cytoplasm 
which  occur  during  development.  But  there 
is  not  a  particle  of  evidence  that  the  division 
of  chromosomes  is  ever  differential;  on  the 
contrary,  there  is  the  most  complete  evidence 
that  their  division  is  always  remarkably  equal 
both  quantitatively  and  qualitatively.  If 
daughter  chromosomes  and  nuclei  ever  become 
unlike,  as  they  sometimes  do,  this  unlikeness 
occurs  long  after  division  and  is  probably  the 
result  of  the  action  of  different  kinds  of  cyto- 
plasm upon  the  nuclei,  as  is  true,  for  example, 
in  the  differentiation  of  the  chromosomes  in 


126  HEREDITY  AND  ENVIRONMENT 

the  somatic  cells  as  contrasted  with  the  germ 
cells  of  Ascaris  (Fig.  30).  But  while  the 
chromosomes  invariably  divide  equally,  other 
portions  of.  the  nucleus  may  not  do  so. 
Achromatin  and  oxychromatin,  like  the  cyto- 
plasm, may  divide  unequally  and  differenti- 
ally, and  this  is  probably  a  prime  factor  in 
development. 

On  the  other  hand,  the  differential  division 
of  the  cytoplasm  is  a  regular  and  character- 
istic feature  of  ontogeny;  indeed,  the  segrega- 
tion and  isolation  of  different  kinds  of 
cytoplasm  in  different  cells  is  the  most  im- 
portant function  of  cell  division  during  de- 
velopment. Thus  we  find  in  the  division 
apparatus  of  the  cell  a  mechanism  for  the  pres- 
ervation in  unaltered  form  of  the  species 
plasm,  or  germ-plasm  of  the  nucleus,  and  for 
the  progressive  differentiation  of  the  personal 
plasm  or  somatoplasm  of  the  cell  body. 

3.  The  Origin  of  the  Sex  Cells.— The  sex 
cells  are  the  latest  of  all  cells  of  a  developing 
organism  to  reach  maturity,  and  yet  they  may 
be  among  the  earliest  to  make  their  appear- 
ance. Every  sex  cell,  like  every  other  type  of 


FlG.    30.       DIFFERENTIATION      OF      GERM      CELLS      AND      SOMATIC 

CELLS  IK  THE  EGG  OF  Ascaris.  A  and  B,  second  cleavage 
division  showing  that  the  chromosomes  remain  entire  in  the 
lower  cell,  which  is  in  the  line  of  descent  of  the  sex  cells 
("germ  track"),  but  that  they  throw  off  their  ends  and  break 
up  into  small  granules  in  the  upper  cells,  which  become 
somatic  cells.  C,  4-cell  stage,  the  nuclei  in  the  upper  (somatic) 
cells  being  small  and  the  ends  of  the  chromosomes  remaining 
as  chromatic  masses  in  the  cell  body  outside  of  the  nuclei, 
while  the  nuclei  in  the  lower  cells  are  much  larger  and  con- 
tain all  the  chromatin.  D,  third  nuclear  division,  showing  the 
somatic  differentiation  of  the  chromosomes  in  all  the  cells 
except  the  lower  right  one,  which  alone  is  in  the  germ  track 
and  will  ultimately  give  rise  to  sex  cells.  (After  Boveri.) 


128  HEREDITY  AND  ENVIRONMENT 

cell,  is  a  lineal  descendant  of  the  fertilized  egg 
( Fig.  22 ) ,  but  the  period  at  which  the  sex  cells 
become  visibly  different  from  other  cells  varies 
from  the  first  cleavage  of  the  egg  in  some 
species  to  a  relatively  advanced  stage  of  de- 
velopment in  others. 

(a)  The  Division  Period.  Oogonia  and 
Spermatogonia. — When  the  primitive  sex 
cells  are  first  distinguishable  they  differ  from 
other  cells  only  in  the  fact  that  they  are  less 
differentiated;  they  have  relatively  larger 
nuclei  and  smaller  cell  bodies,  a  condition 
which  is  indicative  of  little  differentiation  of 
the  cell  body  since  the  products  of  differentia- 
tion such  as  fibres,  secretions,  etc.,  swell  the 
size  of  the  cell  body  but  do  not  contribute  to 
the  growth  of  the  nucleus.  These  primitive 
sex  cells  or  gonia  divide  repeatedly,  but  the 
oogonia  grow  more  rapidly  and  divide  less 
frequently  than  the  spermatogonia.  As  a  re- 
sult of  this  difference  in  the  rate  of  growth 
and  division  the  spermatogonia  become  much 
smaller  and  immensely  more  numerous  than 
the  oogonia.  This  period  in  the  genesis  of  the 
sex  cells  is  known  as  the  division  period 
(Fig.  22), 


THE  CELLULAR  BASIS  129 

(b)  The  Growth  Period.  Oocytes  and 
Spermatocytes. — This  period  of  rapid  cell  di- 
vision is  followed  by  a  period  of  growth  with- 
out division  during  which  the  developing  sex 
cells  are  called  primary  oocytes  or  spermato- 
cytes.  This  growth  period  may  be  very  long 
in  the  case  of  the  oocytes,  lasting,  for  example, 
in  the  human  female  from  the  time  of  birth  to 
the  end  of  the  reproductive  period;  during 
this  long  time  the  oocytes  in  the  ovary  prob- 
ably never  divide,  there  are  as  many  of  them 
at  birth  as  at  any  later  time ;  during  this  period 
of  growth  the  ovarian  egg  becomes  relatively 
large,  in  some  animals,  e.  g.  birds,  the  largest 
of  all  cells.  The  growth  period  of  a  sper- 
matocyte  lasts  for  a  briefer  time  than  does 
that  of  an  oocyte  so  that  the  former  remains 
relatively  small  (Fig.  22). 

All  of  the  cell  divisions  which  take  place 
during  the  division  period  are  of  the  usual 
kind,  in  which  every  chromosome  splits 
lengthwise  into  two  and  the  two  halves  then 
separate  and  move  to  opposite  poles  of  the 
spindle  where  they  break  up  into  threads  and 
granules  and  form  the  daughter  nuclei,  as  is 


130 


HEREDITY  AND  ENVIRONMENT 


shown  in  Fig.  24.  But  during  the  growth 
period  of  the  oocytes  and  spermatocytes  the 
chromosomes  form  a  closely  wound  coil  of 
long  chromatin  threads  (Fig.  31),  and  when 


FIG.  31.  DIFFERENT  STAGES  IN  THE  DEVELOPMENT  OF  THE 
EGG  OF  THE  RABBIT.  A,  at  the  beginning  of  the  growth 
period  showing  slender  chromatic  threads  in  the  nucleus;  B, 
later  stage  in  which  these  threads  ball  up  and  parallel  threads 
conjugate  forming  the  shorter,  thicker  thread  shown  in  C; 
D  and  E,  segmentation  of  the  long  thread  into  chromosomes 
each  of  which  shows  its  double  nature;  F,  later  stage  in 
which  the  distinctness  of  the  chromosomes  is  temporarily  lost, 
(After  Winiwarter.) 


THE  CELLULAR  BASIS  131 

these  threads  uncoil  later  it  is  seen  that  the 
chromosomes  have  united  in  pairs  (Figs.  31  D 
and  Ef  32  B,  33  B )  ;  this  process  is  known  as 
synapsis,  or  the  conjugation  of  the  chromo- 
somes, and  there  is  evidence  that  one  member 
of  each  synaptic  pair  is  derived  from  the 
father,  and  the  other  from  the  mother.  The 
union  of  these  chromosomes  is  probably  not 
so  close  that  they  lose  their  identity,  though 
there  may  possibly  be  some  interchange  of 
substance  between  them.  By  this  union  of  the 
chromosomes  into  pairs  the  number  of  sepa- 
rate chromosomes  is  reduced  to  half  the 
normal  number ;  if  there  are  usually  4  chromo- 
somes, as  in  Ascaris,  they  are  reduced  to  2 
pairs ;  if  48  chromosomes,  as  in  man,  there  are 
24  of  these  pairs. 

In  the  conjugation  of  the  chromosomes  it 
is  plain  that,  generally  speaking,  those 
chromosomes  which  are  similar  in  shape  and 
size  unite;  big  chromosomes  unite  with  big 
ones,  little  ones  with  little  ones,  and  those  of 
peculiar  shape  with  others  of  similar  shape 
(Figs.  32  E,  33  B).  It  is  probable  that  the 
two  members  of  a  pair  of  conjugating  chromo- 


132  HEREDITY  AND  ENVIRONMENT 

A 


FIG.  32.  SPERMATOGENESIS  OF  A  NEMATODE  WORM  (Ancyra- 
canthus).  A,  chromosomes  of  sperm  mother  cell,  11  in  num- 
ber, before  their  union  into  pairs.  B,  early  stage  of  first 
division;  10  of  the  chromosomes  have  united  into  5  pairs  and 
each  of  these  has  split  lengthwise;  1  chromosome  remains  un- 


THE  CELLULAR  BASIS  133 

somes  are  homologous  not  merely  in  shape  and 
size  but  also  in  function,  though  this  homology 
does  not  amount  to  identity. 

In  some  instances  it  can  be  proved  that  one 
member  of  each  conjugating  pair  of  chromo- 
somes comes  from  one  parent  and  the  other 
from  the  other  parent,  and  it  is  probable  that 
this  is  always  true.  In  every  cell  of  every 
individual  which  has  developed  from  a  ferti- 
lized egg  there  are  two  full  sets  of  chromo- 
somes, one  of  which  came  from  the  sperm  and 
the  other  from  the  egg;  but  when  this  indi- 
vidual in  its  turn  produces  germ  cells  homolo- 
gous chromosomes  of  each  set  unite  in  pairs, 
during  the  growth  period. 

These  synaptic  pairs  are  the  bivalent  chrom- 
osomes, and  in  addition  to  showing  the  line  of 
junction  by  which  they  are  united  they  f re- 


paired. C,  first  maturation  division  after  the  5  pairs  of  chrom- 
osomes have  pulled  apart;  the  unpaired  chromosome  is  going 
entire  to  one  pole  of  the  spindle.  D,  two  cells  resulting  from 
this  division,  one  containing  5  and  the  other  6  chromosomes. 
E,  four  cells  resulting  from  the  division  of  two  cells  like  D,  in 
which  each  chromosome  has  split  into  two  so  that  two  of 
the  cells  contain  5  and  two  contain  6  chromosomes.  F,  two 
of  these  cells  changing  into  spermatozoa,  one  containing  5  and 
the  other  6  chromosomes.  (After  Mulsow.) 


124  HEREDITY  AND  ENVIRONMENT 

A  ? 


FIG.  33.     For  description  see  p.  135. 


THE   CELLULAR  BASIS  135 

quently  show  a  longitudinal  split  through  the 
middle  of  each  chromosome  and  at  right  angles 
to  the  line  of  junction.  It  thus  happens  that 
these  bivalent  chromosomes  are  frequently 
four-parted  and  such  four-parted  chromo- 
somes are  known  as  tetrads  (Figs.  32  B, 
33  B). 

(c)  The  Maturation  Period. — Finally  at 
the  close  of  the  growth  period  both  oocyte  and 
spermatocyte  undergo  two  peculiar  divisions, 
one  following  immediately  after  the  other, 
which  are  unlike  any  other  cell  divisions. 
These  are  known  as  the  first  and  second  ma- 
turation divisions  and  they  are  the  last  divi- 
sions which  take  place  in  the  formation  of  the 


FIG.  33.  OOGENESIS  OF  A  NEMATODE  WORM  (Ancyracanthus) . 
A,  egg  mother  cell  containing  12  chromosomes  before  their 
union  into  pairs.  B,  early  stage  of  first  maturation  division; 
all  the  chromosomes  have  united  into  6  pairs,  and  all  but  one 
of  these  has  split  in  two  so  that  the  pairs  are  really  four-parted 
(tetrads).  C,  the  six  tetrads  in  the  first  maturation  division. 
D,  egg  containing  6  chromosomes,  after  both  first  and  second 
maturation  divisions;  the  eliminated  chromosomes  are  shown 
as  the  polar  bodies  at  the  margin  of  the  eggs.  E  and  F,  eggs 
after  fertilization;  the  egg  nucleus  is  above  and  contains  6, 
chromosomes,  the  sperm  nucleus  is  below  and  contains  5  chro- 
mosomes in  one  case  and  6  in  the  other;  the  former  becomes  a 
male  with  11  chromosomes,  the  latter  a  female  with  12 
chromosomes.  (After  Mulsow.) 


136  HEREDITY  AND  ENVIRONMENT 

egg  and  sperm.  In  one  or  the  other  of  these 
two  maturation  divisions  the  pairs  of  chromo- 
somes separate  along  the  line  of  junction,  one 
member  of  each  pair  going  to  one  pole  of  the 
spindle  and  the  other  to  the  other  pole,  so  that 
in  each  of  the  daughter  cells  thus  formed 
only  a  single  set  of  chromosomes  is  present 
(Fig.  32  C  and  D)  ;  but  since  the  position  of 
the  pairs  of  chromosomes  in  the  spindle  is  a 
matter  of  chance  it  rarely  happens  that  all 
the  paternal  chromosomes  go  to  one  pole  and 
all  the  maternal  ones  to  the  other;  thus  each 
of  the  sex  cells  comes  to  contain  a  complete 
set  of  chromosomes,  though  particular  indi- 
vidual chromosomes  may  have  come  from  the 
father  while  others  have  come  •  from  the 
mother.  There  is  reason  to  believe  that 
homologous  chromosomes  show  general  re- 
semblances but  individual  differences,  and  con- 
sequently when  the  members  of  each  pair 
separate  and  go  into  the  sex  cells  these  cells 
differ  among  themselves  because  the  individual 
'chromosomes  in  different  cells  are  not  the 
same  in  hereditary  value. 

In  this  way  the  number  of  chromosomes  in 


THE  CELLULAR  BASIS  13? 

the  mature  egg  or  sperm  comes  to  be  one-half 
the  number  present  in  other  kinds  of  cells, 
and  when  the  egg  and  sperm  unite  in  fertili- 
zation the  whole  number  is  again  restored. 
The  double  set  of  chromosomes  is  known  as 
the  diploid  number,  the  single  set  as  the  hap- 
loid  number,  and  the  maturation  division  in 
which  this  reduction  from  the  double  to  the 
single  set  takes  place  is  the  reduction  division. 
It  is  generally  held  that  this  reduction  takes 
place  in  the  first  of  the  two  maturation  di- 
visions (Fig.  32,  C,  D),  and  that  the  second 
of  these  divisions  is  like  an  ordinary  mitosis 
in  that  each  chromosome  splits  into  two  and 
the  halves  move  apart,  such  a  division  being 
known  as  an  equation  division  (Fig.  32  E), 
but  it  is  possible  that  some  chromosome  pairs 
undergo  an  equation  division  in  the  first  ma- 
turation mitosis  and  a  reduction  division  in 
the  second,  while  other  chromosome  pairs  may 
reverse  this  order. 

It  is  an  interesting  fact  that  long  before 
the  reduction  of  chromosomes  had  been  actu- 
ally seen  Weismann  maintained  on  theoretical 
grounds  that  such  a  reduction  must  occur, 


138  HEREDITY  AND  ENVIRONMENT 

otherwise  the  number  of  chromosomes  would 
double  in  every  generation,  and  he  held  that 
this  reduction  must  take  place  in  one  of  the 
maturation  divisions;  this  hypothesis  of  Weis- 
mann's  is  now  an  established  fact. 

As  the  result  of  these  two  maturation  divi- 
sions four  cells  are  formed  from  each  cell 
( spermatocyte  or  oocyte)  of  the  growth 
period.  In  the  spermatogenesis  each  of  these 
four  cells  is  transformed  into  a  functional 
spermatozoon  (Fig.  32  E),  by  the  condensa- 
tion of  the  nucleus  into  the  sperm  head  and  the 
outgrowth  of  the  centrosome  and  cytoplasm 
to  form  the  tail.  In  the  oogenesis  only  one 
of  these  four  cells  becomes  a  functional  egg 
while  the  other  three  are  small  rudimentary 
eggs  which  are  called  polar  bodies  and  which 
take  no  further  part  in  development  (Fig.  23, 
C-F).  The  fertilization  of  the  egg  usually 
takes  place  coincidently  with  the  formation  of 
the  polar  bodies,  and  so  we  come  back  once 
more  to  the  stage  from  which  we  started,  thus 
completing  the  life  cycle. 

4.  Sex  Determination. — In  the  formation 
of  the  sex  cells  one  can  frequently  distinguish 


THE  CELLULAR  BASIS  139 

at  an  early  stage  differences  between  the 
larger  oogonia  and  the  smaller  and  more  nu- 
merous spermatogonia ;  this  difference  is  the 
first  visible  distinction  in  the  development  of 
the  two  sexes.  In  the  case  of  the  human 
embryo  this  distinction  can  be  made  as  early 
as  the  fifth  week,  and  it  is  evident  that  the  real 
causes  of  this  difference  must  be  found  at  a 
still  earlier  period  of  development. 

The  cause  of  sex  has  been  a  favorite  subject 
of  speculation  for  thousands  of  years.  Hun- 
dreds of  hypotheses  have  been  advanced  to 
explain  this  perennially  interesting  phenome- 
non. The  causes  of  sex  determination  have 
been  ascribed  to  almost  every  possible  external 
or  internal  influence  and  the  world  is  full  of 
people  who  think  they  have  discovered  by 
personal  experience  just  how  sex  is  deter- 
mined. Unfortunately  these  hypotheses  and 
rules  are  generally  founded  upon  a  few  obser- 
vations of  selected  cases.  Since  there  are  only 
two  sexes  the  chances  are  that  any  hypothesis 
will  be  right  half  the  time,  and  if  only  one  for- 
gets the  failures  of  a  rule  and  remembers  the 
times  when  it  holds  good  it  is  possible  to  be- 


140  HEREDITY  AND  ENVIRONMENT 

lieve  in  the  influence  of  food  or  temperature 
or  age,  of  war  or  peace  or  education  on  the 
relative  numbers  of  the  sexes,  or  on  almost  any 
other  thing.  By  statistics  it  has  been  shown 
that  each  of  these  things  influences  the  sex 
ratio,  and  by  more  extensive  statistics  it  has 
been  proved  that  they  do  not. 

This  was  the  condition  regarding  the  causes 
of  sex  determination  which  prevailed  up  to 
the  year  1902.  Immediately  preceding  that 
year  it  had  been  found  that  two  kinds  of  sper- 
matozoa were  formed  in  equal  numbers  in  cer- 
tain insects;  one  of  these  kinds  contained  a 
peculiar  "accessory"  chromosome,  and  the 
other  lacked  it.  The  manner  in  which  these 
two  types  of  spermatozoa  were  formed  had 
been  carefully  worked  out  by  several  investi- 
gators without  any  suspicion  of  the  real  sig- 
nificance of  the  facts.  It  was  shown  that  an 
uneven  number  of  chromosomes  might  be 
present  in  the  spermatogonia  of  certain  in- 
sects and  that  when  maternal  and  paternal 
chromosomes  united  in  pairs  in  synapsis  one 
"odd"  chromosome  was  .left  without  a  mate 
(Fig.  32  B) .  Later,  in  the  reduction  division, 


THE  CELLULAR  BASIS  141 

when  the  synaptic  pairs  separated,  the  odd 
chromosome  went  entire  into  one  of  the  daugh- 
ter cells,  and  the  spermatozoa  formed  from 
this  cell  contained  one  chromosome  more  than 
those  formed  from  the  other  daughter  cell 
(Fig.  32  C  andD). 

Chiefly  because  these  two  kinds  of  sperma- 
tozoa occur  in  equal  numbers  McClung  in 
1902  concluded  that  this  accessory  chromo- 
some was  a  sex-determinant.  In  1905  Wilson 
discovered  in  a  number  of  bugs  that  while 
there  were  two  types  of  spermatozoa,  one  of 
which  contained  and  the  other  lacked  the  ac- 
cessory chromosome,  there  was  only  one  type 
of  egg,  since  every  egg  contained  the  accessory 
chromosome,  and  he  pointed  out  that  if  an 
egg  were  fertilized  by  a  sperm  containing  an 
accessory,  two  accessories  would  be  present  in 
the  zygote,  this  being  the  condition  of  the 
female,  while  if  it  were  fertilized  by  a  sperm 
without  an  accessory  there  would  be  present 
in  the  zygote  only  the  accessory  derived  from 
the  egg  (Fig.  33  E  and  F,  Fig.  34). 

In  other  cases  Miss  Stevens  as  well  as  Wil- 
son discovered  that  two  accessory  chromo- 


142 


HEREDITY  AND  ENVIRONMENT 


somes,  differing  in  size,  might  be  present  in 
the  male  whereas  in  the  female  they  are  of 
equal  size  (Fig.  35).  In  such  cases  two  types 
of  spermatozoa  are  produced  in  equal  num- 
bers, one  containing  a  large  and  the  other  a 
small  accessory  chromosome,  whereas  every 


Oocyte 


Spe 


FIG.  34.  DIAGRAMS  OF  SEX  DIFFERENTIATION  IN  THE  BUG, 
Protenor.  The  oocyte  contains  6  chromosomes  arid  the  sper- 
matocyte  5  chromosomes  which  are  not  yet  united  into  synap- 
tic  pairs;  the  "sex"  chromosomes  are  shown  in  black,  two  are 
present  in  the  oocyte,  but  only  one  in  the  spermatocyte.  In 
the  reduction  division  the  synaptic  pairs  separate,  giving  rise 
to  two  types  of  spermatozoa,  one  of  which  has  the  sex  chrom- 
osome and  the  other  lacks  it;  all  ova  are  alike  in  this  regard. 
If  an  egg  is  fertilized  by  a  sperm  without  the  sex  chrom- 
osome a  male  results;  if  fertilized  by  a  sperm  containing  the 
sex  chromosome  a  female  results.  (After  Wilson  with 
modifications.) 


THE   CELLULAR   BASIS 


143 


egg  contains  one  large  accessory  chromosome. 
If  such  an  egg  is  fertilized  by  a  sperm  con- 
taining a  large  accessory  (the  X  chromosome) 
it  gives  rise  to  a  female,  if  by  a  sperm  contain- 
ing a  small  accessory  (the  Y  chromosome)  it 
gives  rise  to  a  male  (Fig.  35). 

In  other  animals  one  may  not  be  able  to 
distinguish  separate  X  or  Y  chromosomes  and 
yet  such  structures  may  be  joined  to  one  or 
two  ordinary  chromosomes.  This  is  the  case 


Oocyte 


Reduotio 
Divisia 


Spermatoeyte 


FIG.  35.  DIAGRAMS  OF  SEX  DIFFERENTIATION  IN  THE  BEETLE, 
Tenebrio,  showing  5  synaptic  pairs  of  chromosomes  (there  are 
actually  10  pairs)  ;  in  the  oocyte  the  members  of  each  pair  are 
equal  in  size;  in  the  spermatocyte  the  members  of  one  pair 
are  unequal.  These  pairs  separate  in  the  reduction  division 
giving  rise  to  two  types  of  spermatozoa  and  one  type  of  ova; 
eggs  fertilized  by  one  type  of  sperm  give  rise  to  females,  those 
fertilized  by  the  other  type  give  rise  to  males.  (After  Stevens 
with  modifications.) 


144 


HEREDITY  AND  ENVIRONMENT 


in  the  thread  worm,  Ascaris  (Fig.  36),  where 
two  such  accessory  elements  are  present  in  the 
female,  each  being  joined  to  the  end  of  an 
ordinary  chromosome,  whereas  in  the  male 
only  one  such  element  is  present.  Here  also 
two  classes  of  spermatozoa  are  found  one  with 
and  the  other  without  the  accessory  element, 
whereas  all  ova  have  this  element,  and  in  this 
case  also  sex  is  probably  determined  by  the 

Ascaris     Type 

Mature   Egg  and 
Polar  Body 


FIG.  36.  DIAGRAMS  OF  SEX  DIFFERENTIATION  IN  THE  THREAD 
WORM,  Ascaris.  The  X  chromosomes  are  here  joined  to 
ordinary  chromosomes,  there  being  two  in  the  egg  mother 
cell  and  one  in  the  sperm  mother  cell.  All  eggs  contain  one 
of  these  X  chromosomes,  while  half  of  the  spermatozoa  have 
it  and  half  do  not.  Eggs  fertilized  by  one  type  of  sperm 
produce  females,  those  fertilized  by  the  other  type  produce 
males.  (From  Wilson.) 


THE   CELLULAR   BASIS  145 

type  of  spermatozoon  which  enters  the  egg 
(Fig.  36). 

Even  in  man  sex  is  determined  in  the  same 
manner,  according  to  several  recent  investi- 
gators. There  are  in  the  spermatogonia  of 
man  47  chromosomes,  according  to  Winiwar- 
ter,  one  of  which  is  the  X  or  accessory  chromo- 
some (Fig.  37  A).  These  unite  in  synapsis 
into  23  pairs,  leaving  the  X  chromosome  un- 
paired (Fig.  37,  B)  and  in  the  reduction 
division  the  pairs  separate,  while  the  X  chro- 
mosome goes  entire  into  one  of  the  daughter 
cells,  which  consequently  contains  23  +  X 
chromosomes,  whereas  the  other  daughter  cell 
contains  23  chromosomes  (Fig.  37  C  and  D). 
The  former  gives  rise  to  spermatozoa  with  24 
chromosomes,  the  latter  to  spermatozoa  with 
23  chromosomes.  In  the  female  there  are 
probably  48  chromosomes,  according  to  Wini- 
warter,  there  being  two  X  chromosomes,  one 
from  each  parent,  and  after  the  reduction  di- 
visions every  egg  contains  24  chromosomes.  If 
an  egg  is  fertilized  by  a  sperm  containing  24 
chromosomes  an  individual  with  48  chromo- 
somes, or  a  female,  is  produced;  if  fertilized 


146 


HEREDITY  AND  ENVIRONMENT 


by  a  sperm  with  23  chromosomes  an  individual 
with  47  chromosomes,  or  a  male,  results 
(Fig.  37). 


FIG.  37.  DIAGRAMS  OF  SEX  DIFFERENTIATION  IN  MAN.  A, 
spermatogonium  with  47  chromosomes  one  of  which  (small 
circle)  is  the  X  chromosome.  B,  spermatocyte  showing  23  syn- 
aptic  pairs  and  a  single  unpaired  X  chromosome.  C,  reduction 
division  in  which  the  synaptic  pairs  separate  while  the  X 
chromosome  does  not  divide,  consequently  the  second  sper- 
matocytes  D  and  D'  contain  respectively  23  _|-  X  and  23 
chromosomes.  E  and  E',  second  maturation  division  in  which 
every  chromosome  divides,  giving  rise  to  two  equal  classes  of 
spermatids  and  spermatozoa,  one  of  which  has  24  chromo- 
somes and  the  other  23.  If  an  egg  containing  24  chromo- 
somes is  fertilized  by  a  sperm  with  24,  a  female  with  48 
chromosomes  is  produced;  if  an  egg  with  24  chromosomes  is 
fertilized  by  a  sperm  with  23,  a  male  with  47  chromosomes 
results.  (From  Morgan  after  Winiwarter.) 


THE  CELLULAR  BASIS  147 

It  must  be  said  that  other  investigators, 
notably  Guyer  and  Montgomery,  have  not 
found  47  chromosomes  in  the  spermatogonia 
of  man,  but  22.  Since  both  the  latter  investi- 
gators worked  on  negroes  whereas  Winiwar- 
ter  worked  on  white  men  it  has  been  suggested 
quite  recently  by  Morgan  and  Guyer  that 
there  may  be  twice  as  many  chromosomes  in 
the  white  race  as  in  the  black.  A  similar  con- 
dition in  which  one  race  has  twice  as  many 
chromosomes  as  another  race  of  the  same 
species  is  found  in  two  races  of  the  thread 
worm,  Ascaris  megalocephala,  but  it  is  still  too 
soon  to  affirm  that  this  is  true  of  white  and 
black  races  of  man. 

Similar  correlations  between  chromosomes 
and  sex  have  been  observed  in  more  than  one 
hundred  species  of  animals  belonging  to  widely 
different  phyla.  In  a  few  classes  of  animals, 
particularly  echinoderms  and  birds,  the  evi- 
dence while  not  entirely  convincing  seems  to 
point  to  the  fact  that  two  types  of  ova  are 
produced  and  but  one  type  of  spermatozoa; 
but  the  general  principle  that  sex  is  determined 
by  the  chance  union  of  male-producing  or  fe- 


148  HEREDITY  AND  ENVIRONMENT 

male-producing  gametes  is  not  changed  by 
such  cases. 

On  the  other  hand  there  are  many  observa- 
tions which  seem  to  indicate  that  the  sex  ratio 
may  be  changed  by  environmental  conditions 
acting  before  or  after  fertilization  and  that 
therefore  sex  is  determined  by  extrinsic  rather 
than  by  intrinsic  causes.  Many  of  these  ob- 
servations, as  already  remarked,  are  now 
known  to  be  erroneous  or  misleading,  since 
they  do  not  prove  what  they  were  once  sup- 
posed to  demonstrate.  But  there  remain  a 
few  cases  which  can  not  at  present  be  ex- 
plained away  in  this  manner.  Perhaps  the 
best  attested  of  these  are  the  observations  of 
R.  Hertwig  and  some  of  his  pupils  on  the 
effects  of  the  time  of  fertilization  on  the  de- 
termination of  sex.  If  frog's  eggs,  which  are 
always  fertilized  after  they  are  laid,  are  kept 
for  some  hours  before  spermatozoa  are  mixed 
with  them,  or  if  the  female  is  prevented  for 
two  or  three  days  from  laying  the  eggs  after 
they  have  entered  the  oviducts,  the  proportion 
of  males  to  females  is  enormously  increased. 
A  wholly  similar  result  has  been  reached  by 


THE  CELLULAR  BASIS  149 

Pearl  and  Parshley  in  the  case  of  cattle,  where 
delayed  fertilization  of  the  egg  leads  to  a  great 
preponderance  of  males.  Hertwig  attempts 
to  explain  his  extremely  interesting  and  im- 
portant observations  as  due  to  the  relative  size 
of  nucleus  and  cytoplasm  of  the  egg;  but  in 
general  this  nucleus-plasma  ratio  may  vary 
greatly  irrespective  of  sex  and  there  is  no 
clear  evidence  that  it  is  a  cause  of  sex 
determination. 

Miss  King,  also  working  on  toad's  eggs,  has 
increased  the  proportion  of  females  by  slightly 
drying  the  eggs  or  by  withdrawing  water  from 
them  by  placing  them  in  solutions  of  salts, 
acids,  sugar,  etc.,  but  the  manner  in  which 
drying  increases  the  proportion  of  females  is 
wholly  unknown. 

Extensive  statistics  show  that  in  many  ani- 
mals including  man  more  males  are  born  than 
females  whereas  according  to  the  chromosome 
theory  of  sex  determination  as  many  female- 
producing  spermatozoa  are  formed  as  male- 
producing  ones.  It  is  possible  to  explain  such 
departure  from  the  1 : 1  ratio  of  males  and  fe- 
males in  conformity  with  the  chromosome 


150  HEREDITY  AND  ENVIRONMENT 

theory  if  one  class  of  spermatozoa  are  more  ac- 
tive or  have  greater  vitality  than  the  other 
class,  or  if  after  fertilization  one  sex  is  more 
likely  to  live  than  the  other.  In  the  human 
species  it  is  known  that  mortality  is  greater 
in  male  babies  before  and  after  birth  than  in 
female  babies,  and  if  before  fertilization  the 
activity  or  vitality  of  male-producing  sper- 
matozoa is  greater  than  of  female-producing 
ones  it  would  offer  a  possible  explanation  of 
the  greater  number  of  males  than  of  females 
at  the  time  of  birth.  In  certain  insects  it  is 
known  that  only  females  develop  from  ferti- 
lized eggs,  and  in  one  of  these  cases,  viz., 
Phylloxera,  Morgan  has  discovered  that  this 
is  due  to  the  fact  that  all  the  male-producing 
spermatozoa  degenerate  and  that  only  female- 
producing  spermatozoa  become  functional. 
Possibly  experimental  alterations  of  the  sex 
ratio,  such  as  Hertwig,  King  and  others  have 
brought  about  may  be  explained  in  a  similar 
way.  At  least  the  chromosomal  theory  of  sex 
determination  is  so  well  supported  in  so  many 
cases  and  has  been  found  to  apply  in  so  many 
instances  where  at  first  it  seemed  impossible  of 


THE   CELLULAR  BASIS  151 

application,  that  it  ought  not  to  be  abandoned 
until  unmistakable  evidence  can  be  adduced 
against  it.  For  the  present  at  least  we  are 
justified  in  concluding  that  sex  is  irrevocably 
determined  at  the  time  of  fertilization. 

C.  THE  MECHANISM  OF  HEREDITY 
The  mechanism  of  heredity,  as  contrasted 
with  the  mechanism  of  development,  consists 
in  the  formation  of  particular  kinds  of  germ 
cells  and  in  the  union  of  certain  of  these  cells 
in  fertilization.  We  have  briefly  traced  the 
origin,  maturation  and  union  of  male  and  fe- 
male sex  cells  in  a  number  of  animals,  and  in 
these  phenomena  we  have  the  mechanism  of 
the  hereditary  continuity  between  successive 
generations.  But  in  addition  to  these  specific 
facts  there  are  certain  general  considerations 
which  need  to  be  emphasized. 

I.  THE  SPECIFICITY  or  GERM  CELLS 

The  conclusion  is  inevitable  that  the  germ 
cells  of  different  species  and  even  those  of  dif- 
ferent individuals  are  not  all  alike.  Every  in- 
dividual difference  between  organisms  must 


152  HEREDITY  AND  ENVIRONMENT 

be  due  to  one  or  more  differentiating  causes 
or  factors.  Specific  results  come  only  from 
specific  causes.  These  causes  may  be  found  in 
the  organization  of  the  germ  cells  or  in  en- 
vironmental stimuli,  i.  e.,  they  may  be  intrinsic 
or  extrinsic,  but  as  a  matter  of  fact  experience 
has  shown  that  they  are  generally  intrinsic  in 
the  germ.  In  the  same  environment  one  egg 
becomes  a  chicken  and  another  a  duck;  one 
becomes  a  frog,  and  another  a  fish,  and  an- 
other a  snail;  one  becomes  a  black  guinea-pig 
and  another  a  white  one;  one  becomes  a  male 
and  another  a  female;  one  gives  rise  to  a  tall 
man  and  another  to  a  short  man,  etc.  Since 
these  differences  may  occui'  in  the  same  en- 
vironment they  must  be  due  to  differences  in 
the  germ  cells  concerned. 

On  the  other  hand  different  environmental 
conditions  may  be  associated  with  similar  de- 
velopmental results.  Loeb  and  others  have 
found  that  artificial  parthenogenesis  may  be 
induced  by  a  great  variety  of  environmental 
stimuli,  viz.,  by  salt  solutions,  by  acids  and 
alkalis,  by  fatty  acids  and  fat  solvents,  by 
alkaloids  and  cyanides,  by  blood  serum  and 


THE  CELLULAR  BASIS  153 

sperm  extract,  by  heat  and  cold,  by  agitation 
and  electric  current.  There  is  certainly  noth- 
ing specific  in  these  different  stimuli.  Simi- 
larly Stockard  has  discovered  that  cyclopia, 
or  one-eyed  monsters,  may  be  produced  by 
magnesium  salts,  alcohol,  chloretone,  chloro- 
form, and  ether.  In  all  such  cases  it  is  evident 
that  the  specific  results  of  such  treatment  are 
due  to  a  specific  organization  of  the  germ 
rather  than  to  specific  stimuli. 

Why  does  one  egg  give  rise  to  a  chicken 
and  another  to  a  duck,  or  a  fish,  or  a  frog? 
Why  does  one  egg  give  rise  to  a  black  guinea- 
pig  and  another  to  a  white  one,  though  both 
may  be  produced  by  the  same  parents?  Why 
does  one  child  differ  from  another  in  the  same 
family  ?  Why  does  one  cell  give  rise  to  a  gland 
and  another  to  a  nerve,  one  to  an  egg  and  an- 
other to  a  sperm?  If  these  differences  are  not 
due  to  environmental  causes,  and  the  evidence 
shows  that  they  are  not,  they  must  be  due  to 
differences  in  the  structures  and  functions  of 
the  cells  concerned. 

Many  differences  in  the  material  substances 
of  cells  are  visible,  and  many  more  are  invisi- 


154  HEREDITY  AND  ENVIRONMENT 

ble  though  still  demonstrable.  These  differ- 
ences may  not  be  detectable  by  chemical  or 
physical  tests,  and  yet  they  may  be  demon- 
strated physiologically  and  development  ally. 
The  most  delicate  of  all  tests  are  physiological, 
as  is  shown  by  the  Weidal  test  in  typhoid 
fever,  the  Wassermann  reaction  in  syphilis, 
the  reactions  of  immunized  animals  to  different 
toxins,  etc.  Lillie  has  recently  shown  that 
egg  cells  give  off  a  substance  which  he  calls 
fertilizin,  which  can  be  detected  only  by  the 
way  in  which  spermatozoa  react  to  it.  No 
chemical  or  physical  test  can  distinguish  be- 
tween the  different  eggs  or  spermatozoa  pro- 
duced by  the  same  individual,  but  the  reactions 
of  these  cells  in  development  prove  that  they 
are  different.  Undoubtedly  chemical  and 
physical  differences  are  here  present  but  no 
chemical  methods  at  present  available  are 
sufficiently  delicate  to  detect  them.  The  de- 
velopmental test  proves  that  there  must  be  as 
many  kinds  of  germs  as  there  are  different 
kinds  of  individuals  which  come  from  germs. 
It  is  one  of  the  marvellous  facts  of  biology 
that  every  individual  which  has  been  produced 


THE   CELLULAR  BASIS  155 

sexually  is  unique,  the  first  and  last  of  its 
identical  kind,  and  although  some  of  these  in- 
dividual differences  are  due  to  varying  en- 
vironment, others  are  evidently  due  to 
germinal  differences,  so  that  we  must  conclude 
that  every  fertilized  egg  cell  differs  in  some 
respects  from  every  other  one. 

But  are  there  molecules  and  atoms  enough 
in  a  tiny  germ  cell,  such  as  a  spermatozoon, 
to  allow  for  all  these  differences?  Miescher 
has  shown  that  a  molecule  of  albumin  with  40 
carbon  atoms  may  have  as  many  as  one  billion 
stereoisomers,  and  in  protoplasm  there  are 
many  kinds  of  albumin  and  other  proteins, 
some  with  probably  more  than  700  carbon 
atoms.  In  such  a  complex  substance  as  proto- 
plasm the  possible  variations  in  molecular  con- 
stitution must  be  well  nigh  infinite,  and  it  can 
not  be  objected  on  this  ground  that  it  is 
chemically  and  physically  impossible  to  have  as 
many  varieties  of  germ  cells  as  there  are  dif- 
ferent kinds  of  individuals  in  the  world. 

Even  with  regard  to  morphological  elements 
which  may  be  seen  with  the  microscope  it  can 
be  shown  that  an  enormous  number  of  permu- 


156  HEREDITY  AND  ENVIRONMENT 

tations  is  possible.  It  seems  probable,  as 
Boveri  has  shown,  that  different  chromosomes 
of  the  fertilized  egg  differ  in  hereditary  po- 
tencies, and  where  the  number  of  chromosomes 
is  fairly  large  the  number  of  possible  combi- 
nations of  these  chromosomes  in  the  germ  cells 
becomes  very  great.  In  woman,  where  there 
are  probably  48  chromosomes,  and,  after 
synapsis,  24  pairs  of  maternal  and  paternal 
ones,  the  possible  number  of  permutations 
in  the  distribution  of  these  chromosomes  to 
the  different  egg  cells  would  be  224,  or 
16,777,036,  and  the  possible  number  of  differ- 
ent types  of  fertilized  eggs  or  oosperms  which 
could  be  produced  by  a  single  pair  of  parents 
would  be  ( 16,777,036) 2,  or  approximately 
three  hundred  thousand  billions.  But  prob- 
ably other  things  than  chromosomes  differ  in 
different  germ  cells,  and  it  is  by  no  means 
certain  that  individual  chromosomes  are  al- 
ways composed  of  the  same  chromomeres,  or 
units  of  the  next  smaller  order,  and  in  view  of 
these  possibilities  it  may  well  be  that  every  hu- 
man germ  cell  differs  morphologically  and 
physiologically  from  every  other  one,  in  short 


THE  CELLULAR  BASIS  157 

that  every  oosperni  and  every  individual  which 
develops  from  it  is  absolutely  unique. 

Indeed  the  production  of  unique  individuals 
seems  to  be  the  chief  purpose  and  result  of 
sexual  reproduction.  In  asexual  reproduc- 
tion the  individual  variations  which  occur  are 
chiefly  if  not  entirely  due  to  environment, 
but  in  sexual  reproduction  they  are  also  due  to 
new  combinations  of  hereditary  elements. 
The  particular  germinal  organization  trans- 
mitted from  one  generation  to  the  next  de- 
pends upon  (a)  the  ancestral  organization, 

(b)  the  particular  character  of  the  cell  di- 
visions by  which  the  germ  cells  are  formed, 

(c)  the  particular  kinds  of  egg  and  sperm 
cells  which  combine  in  fertilization.     The  an- 
cestral organization  determines  all  the  gen- 
eral  characteristics    of   race,    species,    genus, 
order,  phylum.     It  determines  the  possibili- 
ties and  limitations  of  individual  variations. 
Given   a   certain   ancestral   organization,   the 
individual  peculiarities  of  the  germ  cells  are 
determined  by  the  particular  character  of  cell 
division  by  which  the  germ  cells  are  formed, 
and  the  peculiarities  of  the  individuals  or  per- 


158  HEREDITY  AND  ENVIRONMENT 

sons  which  develop  from  these  cells  are  de- 
termined in  large  part  by  the  particular  kinds 
of  germ  cells  which  unite  in  fertilization. 

The  behavior  of  chromosomes  in  matura- 
tion and  fertilization  is  like  the  shuffle  and  deal 
of  cards  in  a  game,  and  apparently  with  the 
same  object,  viz.,  never  to  deal  the  same 
hand  twice.  To  make  this  comparison  more 
complete  suppose  that  kings  be  discarded  from 
the  pack,  leaving  48  cards  of  two  colors,  red 
and  black,  which  we  will  compare  to  the  48 
chromosomes  of  maternal  and  paternal  origin 
in  the  human  oocyte ;  suppose  that  in  the  shuf- 
fling of  these  cards  corresponding  cards  of  the 
red  and  the  black  suits  are  temporarily  stuck 
together  so  that  the  ace  of  diamonds  is  united 
with  the  ace  of  clubs,  the  queen  of  hearts  with 
the  queen  of  spades,  etc.,  thus  forming  24  red- 
black  pairs  of  the  same  denominations.  If 
these  cards  are  then  dealt  into  two  hands,  one 
card  of  each  pair  going  to  one  hand  and  the 
other  to  the  other  hand,  we  will  have  two 
cards  of  each  denomination  in  each  hand,  but 
if  the  cards  are  dealt  indiscriminately  some  of 
them  will  be  red  and  some  black.  This  de- 


THE  CELLULAR  BASIS  159 

scription  parallels  what  takes  place  in  the 
maturation  of  the  human  ovum,  except  that 
there  is  no  evidence  that  there  are  more  than 
two  suits  of  chromosomes,  one  of  which  is  ma- 
ternal and  the  other  paternal. 

To  carry  out  this  comparison  in  the  case  of 
the  maturation  of  the  human  sperm  where 
there  are  only  47  chromosomes  it  is  necessary 
to  take  another  pack  and  discard  an  additional 
card,  say  the  queen  of  clubs ;  then  in  the  union 
of  corresponding  red  and  black  cards  into 
pairs  the  queen  of  hearts  unites  with  the  queen 
of  spades,  but  the  queen  of  diamonds  remains 
alone,  and  when  the  cards  are  dealt  into  two 
hands  as  before  one  hand  will  contain  24  cards 
and  the  other  23. 

If  now  we  complete  this  comparison  by  ex- 
tending it  to  what  takes  place  in  fertilization 
we  must  take  one  hand  from  each  of  these 
deals  and  put  them  together  into  one  pack; 
this  pack  would  contain  cards  of  every  de- 
nomination from  ace  to  queen  but  there  would 
be  varying  numbers  of  red  and  black  cards  and 
a  mixture  of  cards  from  two  distinct  packs. 
In  no  game  of  cards  do  corresponding  cards 


160  HEREDITY  AND  ENVIRONMENT 

from  different  packs  have  slightly  different 
values  nor  are  half  of  the  cards  taken  from  one 
pack  and  half  from  another  at  every  game,  but 
this  is  just  what  happens  in  the  shuffle  and 
deal  of  the  chromosomes.  Because  of  the  mix- 
ture of  chromosomes  from  distinct  individuals 
in  every  generation,  each  of  which  has  its  own 
peculiar  value,  the  game  of  heredity  becomes 
vastly  more  complex  than  any  game  of  cards. 

This  illustration  may  serve  to  make  plain 
the  fact  that  in  the  process  of  maturation  and 
fertilization  there  is  this  shuffle  and  deal  of 
the  chromosomes,  with  the  result  that  every 
oosperm  and  every  individual  which  develops 
from  it  is  different  from  every  other  one. 

This  conception  of  the  specificity  of  every 
germ  cell,  as  well  as  of  every  developed  indi- 
vidual, sets  the  whole  problem  of  heredity  and 
development  in  a  clear  light.  The  visible 
peculiarities  of  an  adult  become  invisible  as 
development  is  traced  back  to  the  germ,  but 
they  do  not  wholly  cease  to  exist.  Similarly 
the  multitudinous  complexities  of  an  adult 
fade  out  of  view  as  development  is  traced  to 
its  earliest  stages,  but  it  is  probable  that  they 


THE   CELLULAR   BASIS  161 

are  not  wholly  lost.  In  short,  the  specificity 
of  the  germ  applies  not  merely  to  those  things 
in  which  it  differs  from  other  germs,  but  also 
to  characters  in  which  it  resembles  others; 
in  short,  to  hereditary  resemblances  no  less 
than  to  hereditary  differences. 

The  mistake  of  the  doctrine  of  preformation 
was  in  supposing  that  germinal  parts  were  of 
the  same  kind  as  adult  parts;  the  mistake  of 
epigenesis  was  in  maintaining  the  lack  of 
specific  parts  in  the  germ.  The  development 
of  every  animal  and  plant  consists  in  the  trans- 
formation of  the  specific  characters  of  the  germ 
into  those  of  the  adult,  but  not  in  the  formation 
of  structures  or  characters  de  novo.  From  be- 
ginning to  end  development  is  a  series  of  mor- 
phological and  physiological  changes  but  not 
of  new  formations  or  creations.  It  is  only  the 
incompleteness  of  our  knowledge  of  develop- 
ment which  allows  us  to  say  that  the  eye  or 
ear  or  brain  begins  to  form  in  this  or  that  stage. 
They  become  visible  at  certain  stages,  but  their 
real  beginnings  are  indefinitely  remote. 


162  HEREDITY  AND  ENVIRONMENT 


II.  CORRELATIONS  BETWEEN  GERMINAL  AND 
SOMATIC  ORGANIZATION 

All  the  world  knows  that  the  organization 
of  the  germ  is  not  the  same  as  that  of  the  de- 
veloped animal  which  comes  from  it,  and  yet 
the  specificity  of  the  germ  indicates  that  there 
must  be  some  correlation  between  the  germinal 
and  the  developed  organization;  in  short, 
there  is  not  identity  of  organization  but  cor- 
relation of  organization  between  the  germ  and 
the  adult.  What  correlations  are  known  to 
exist  between  the  oosperm  and  the  developed 
animal? 

1.  Nuclear  Correlations. — Many  biologists 
maintain  that  the  nucleus  and  more  particu- 
larly the  chromosomes  are  the  exclusive  seat 
of  the  "inheritance  material"  and  that  all  the 
"determiners"  of  adult  characters  are  located 
in  them.  Against  the  extreme  form  of  this 
theory  many  general  and  specific  objections 
may  be  urged.  General  objections  are  based 
upon  the  consideration  that  the  entire  cell, 
cytoplasm  as  well  as  nucleus,  is  concerned  in 


THE   CELLULAR   BASIS  163 

differentiation  and  that  neither  is  capable  of 
embryonic  development  in  the  absence  of  the 
other.  Differentiation  is  indeed  the  result  of 
the  interaction  of  nucleus  and  cytoplasm,  and 
how  then  can  it  be  said  that  the  nucleus  is  the 
only  seat  of  the  inheritance  material?  If  held 
rigidly,  this  theory  involves  the  assumption 
that  the  cytoplasm  and  all  other  parts  of  the 
cell  are  the  products  of  the  chromosomes,  and 
that  therefore  the  chromosome  and  not  the  cell 
is  the  ultimate  independent  unit  of  structure 
and  function;  an  assumption  which  is  con- 
trary to  fact.  Furthermore,  since  heredity  in- 
cludes a  series  of  fundamental  vital  processes 
such  as  assimilation,  growth,  division  and  dif- 
ferentiation, there  is  something  primitive  and 
naive  in  the  view  that  this  most  general  pro- 
cess can  be  localized  in  one  specific  part  of  the 
cell,  something  which  recalls  the  long-past 
doctrine  that  the  life  was  located  in  the  heart 
or  in  the  blood,  or  the  ancient  attempts  to  find 
the  seat  of  the  soul  in  the  pineal  gland  or  in  the 
ventricles  of  the  brain. 

On  the  other  hand  it  is  objected  by  certain 
investigators,  notably  by  Child,  Foot  and  Stro- 


164  HEREDITY  AND  ENVIRONMENT 

bell,  that  chromosomes  are  not  the  causes  of 
anything,  but  that  they  are  the  "results  of 
dynamic  processes,"  "the  expression  rather 
than  the  cause  of  cell  activities."  This  objec- 
tion seems  to  confuse  the  idea  of  natural  cause 
with  that  of  final  cause.  Science  knows  noth- 
ing of  the  latter;  any  natural  cause  is  only  a 
link  in  the  chain  of  cause  and  effect,  it  is  itself 
the  result  of  antecedent  causes  and  the  cause 
of  subsequent  results.  Undoubtedly  the 
chromosomes  are  the  result  of  antecedent  pro- 
cesses, and  yet  they  may  also  be  the  causes  of 
subsequent  processes.  No  thoughtful  person 
has  ever  maintained  that  chromosomes  or  any 
other  things  in  nature  are  autonomous,  abso- 
lute, uncaused  causes. 

There  are  certain  general  and  a  priori  rea- 
sons for  assuming  that  the  chromosomes  are 
important  factors  in  heredity  and  differentia- 
tion: (1)  they  come  in  approximately  equal 
numbers  from  the  father  and  the  mother,  (2) 
one-half  of  each  of  the  maternal  and  paternal 
chromosomes  is  distributed  to  each  cell  of  the 
developing  organism,  (3)  in  the  formation  of 


THE   CELLULAR   BASIS  165 

the  egg  and  sperm  cells  the  normal  number 
of  chromosomes  is  reduced  by  one-half,  and 
(4)  in  fertilization  the  normal  number  is  re- 
stored by  the  union  of  the  chromosomes  of 
the  egg  and  sperm.  It  is  a  remarkable  fact 
that  the  determiners  or  factors  of  certain  in- 
herited characters  come  in  equal  numbers 
from  both  parents  and  that  in  spite  of  their 
ultimate  association  in  an  individual  they  may 
be  separated  or  "segregated"  in  the  formation 
of  that  individual's  germ  cells.  Such  inherit- 
ance is  known  as  Mendelian  and  will  be 
treated  at  length  in  the  next  chapter,  but  it 
may  be  said  here  that  the  association,  distribu- 
tion and  segregation  of  Mendelian  factors 
and  of  maternal  and  paternal  chromosomes 
are  exactly  parallel.  This  is  strong  evidence 
that  these  factors  are  associated  in  some  way 
with  the  chromosomes. 

There  are  also  certain  special  reasons  for 
considering,  that  the  chromosomes  are  import- 
ant factors  in  heredity  and  development:  (5) 
Boveri  has  studied  the  abnormal  distribution 
of  chromosomes  to  different  cleavage  cells  in 
doubly  fertilized  sea  urchin  eggs  and  has 


166  HEREDITY  AND  ENVIRONMENT 

found  evidence  that  the  hereditary  value  of 
different  chromosomes  is  different.  (6)  Mc- 
Clung,  Stevens  and  Wilson  have  discovered 
that  the  determination  of  sex  is  associated 
with  the  presence  or  absence  of  a  particular 
chromosome,  the  X  chromosome,  in  the  sper- 
matozoon which  fertilizes  the  egg.  If  an 
egg  is  fertilized  by  a  sperm  which  lacks  the 
X  chromosome  a  male  is  produced,  if  fertilized 
by  the  other  type  a  female  results.  (7)  Fin- 
ally Morgan  has  found  that  there  is  a  linkage 
of  certain  somatic  characters  with  sex  in  the 
fruit  fly,  Drosophila,  which  can  be  readily  ex- 
plained by  assuming  that  the  determiners  for 
these  characters  are  in  sorae  way  associated 
with  the  sex  chromosome. 

We  have  in  these  facts  a  remarkable  corre- 
lation between  the  distribution  of  the  chromo- 
somes and  the  occurrence  of  certain  characters 
of  the  adult  animal.  The  association  of  ma- 
ternal and  paternal  chromosomes  in  fertiliza- 
tion and  their  segregation  in  the  maturation  of 
the  germ  cells  is  parallel  to  the  association  of 
Mendelian  characters  in  the  zygote  and  their 
segregation  in  the  gametes;  if  the  distribution 


THE   CELLULAR  BASIS  167 

of  chromosomes  in  cleavage  is  abnormal  the 
larva  shows  abnormal  characters  (Boveri)  ; 
sex  determination  is  associated  with  the  distri- 
bution of  a  particular  chromosome  to  one-half 
of  the  spermatozoa,  and  the  fertilization  of  the 
egg  by  one  type  or  the  other  of  spermatozoa 
(Wilson)  ;  the  linkage  of  certain  characters 
with  sex  finds  a  ready  explanation  by  assum- 
ing that  the  differential  causes  of  these  char- 
acters are  associated  with  the  sex  chromosome 
(Morgan). 

2.  Cytoplasmic  Correspondences. — On  the 
other  hand  the  most  direct  and  the  earliest 
recognized  correlations  between  the  oosperm 
and  the  developed  animal  are  found  in  the 
polarity  and  symmetry  of  the  fertilized  egg 
and  of  the  animal  to  which  it  gives  rise. 

(a)  Polarity. — In  all  eggs  there  is  a  polar 
differentiation,  one  pole,  at  which  the  matura- 
tion divisions  take  place,  being  known  as  the 
animal  pole,  and  the  opposite  one  being  known 
as  the  vegetative  pole.  The  substance  of  the 
egg  in  the  vicinity  of  the  animal  pole  usually 
gives  rise  to  the  ectoderm,  or  outer  cell  layer 
of  the  embryo;  the  portion  of  the  egg  sur- 


168  HEREDITY  AND  ENVIRONMENT 

rounding  the  vegetative  pole  usually  becomes 
the  endoderm  or  inner  cell  layer.  The  axis 
which  connects  these  poles,  the  chief  axis  of 
the  egg,  becomes  the  gastrular  axis  of  the  em- 
bryo and  in  every  great  group  of  animals  bears 
a  constant  relationship  to  the  chief  axis  of  the 
adult  animal.  The  polarity  of  the  developed 
animal  is  thus  directly  connected  with  the 
polarity  of  the  egg  from  which  it  came  (Figs. 
23,26,  27,28,  38,39). 

(b)  Symmetry. — In  many  cases  the  sym- 
metry of  the  developed  animal  is  foreshadowed 
in  the  symmetry  of  the  egg.  The  eggs  of 
cephalopods  (Fig.  38)  and  of  insects  (Fig. 
39)  are  bilaterally  symmetrical  while  they  are 
still  in  the  ovary ;  in  other  cases,  such  as  ascid- 
ians,  Amphioxus  and  the  frog,  bilateral  sym- 
metry appears  immediately  after  fertilization 
(Fig.  27,  1,  2),  though  in  some  of  these  cases 
there  is  reason  to  believe  that  the  eggs  are 
bilateral  even  before  fertilization ;  in  still  other 
cases  bilaterality  does  not  become  visible  until 
later  in  development  and  we  do  not  now  know 
whether  it  is  present  in  earlier  stages  or  not; 
but  wherever  it  can  be  recognized  in  the  earlier 


THE   CELLULAR  BASIS 

B 

d 


FIG.  38. 


FIG. 


FIG.  38.  OUTLINES  OF  THE  UNFERTILIZED  EGG  OF  A  SQUID, 
Loligo,  showing  the  polarity  and  symmetry  of  the  egg  with 
reference  to  the  axes  of  the  developed  animal;  d,  dorsal;  v, 
ventral;  /,  left;  r,  right;  a,  anterior;  p,  posterior.  (After 
Watase.) 

FIG.  39.  MEDIAN  SECTION  THROUGH  EGG  OF  A  FLY,  Musca, 
just  after  fertilization,  showing  the  relations  of  the  polarity 
and  symmetry  of  the  egg  to  the  axes  of  the  developed  animal; 
the  long  axis  of  the  egg  corresponds  to  the  antero-posterior 
axis  of  the  animal;  d,  dorsal;  v,  ventral;  TO,  micropyle  through 
which  sperm  enters  the  egg;  g,  glutinous  cap  over  the  micro- 
pyle; r,  polar  bodies;  p,  egg  and  sperm  nuclei;  do,  yolk; 
k,  peripheral  layer  of  protoplasm;  dh,  vitelline  membrane  of 
egg;  ch,  ehorion.  (After  Korschelt  and  Heider.) 


FIGS.  40,  41,  42.  THE  CAUSE  OF  INVERSE  SYMMETRY  IN 
SNAILS.  In  each  case  the  right-hand  column  represents  dextral 
forms,  the  left-hand  column  sinistral  ones. 

FIG.  40.  NORMAL  AND  INVERSE  SYMMETRY  IN  THE  UN- 
SEGMENTED  EGG  AND  IN  THE  FlRST  AND  SECOND  CLEAVAGES. 


THE  CELLULAR  BASIS  171 

stages  it  is  certain  that  the  bilateral  symmetry 
of  the  egg  becomes  the  bilateral  symmetry  of 
the  developed  animal. 

(c)  Inverse  Symmetry. — In  most  animals 
bilateral  symmetry  is  not  perfect,  certain  or- 
gans being  found  on  one  side  of  the  mid  line 
and  not  on  the  other,  or  being  larger  or  dif- 
ferently located  on  one  side  as  compared  with 
the  other;  among  all  such  animals  variations 
occasionally  occur  which  show  a  complete  re- 
versal of  these  asymmetrical  organs,  i.  e.}  in 
man  the  heart  and  arch  of  the  aorta  may  occur 
on  the  right  side  instead  of  the  left,  the  pyloris 
and  chief  portion  of  the  liver  on  the  left  in- 
stead of  the  right,  etc.  Among  certain  snails 
this  inversion  of  symmetry  may  occur  regu- 
larly in  certain  species  and  not  in  others,  the 
inverse  form  being  known  as  sinistral  and  the 
ordinary  form  as  dextral  ( Fig.  42 ).  In  these 
sinistral  snails,  and  probably  in  all  animals 
showing  inverse  symmetry,  the  embryo  is  in- 
versely symmetrical  and  every  cleavage  of  the 
egg  from  the  first  to  the  last  is  the  inverse  of 
that  which  occurs  in  dextral  snails  (Figs.  40, 
41).  There  is  good  reason  to  believe  that  in 


172  HEREDITY  AND  ENVIRONMENT 

such  cases  the  unsegmented  egg  is  also  in- 
versely symmetrical  as  compared  with  the 
more  usual  type  (Fig.  40).  In  all  of  these 
cases  there  is  a  direct  correspondence  between 
the  polarity  and  symmetry  of  the  oosperm 
and  the  polarity  and  symmetry  of  the  devel- 
oped animal  (Figs.  38-42). 

(d)  Localization  Pattern. — In  many  ani- 
mals the  ectoderm,  endoderm  and  mesoderm 
may  be  traced  back  to  areas  of  peculiar  proto- 
plasm in  the  oosperm,  but  in  addition  to  this 
one  can  recognize  in  the  ascidian  egg  areas  of 
peculiar  protoplasm  which  will  give  rise  to 
mesenchyme,  muscles,  nervous  system  and 
notochord,  and  these  substances  are  present 
in  the  oosperm  in  the  approximate  positions 
and  proportions  which  they  will  have  in  the 
embryo  and  larva  (Figs.  27-29). 

Indeed  there  are  types  of  localization  of 
these  cytoplasmic  materials  in  the  egg  which 
are  characteristic  of  certain  phyla;  thus  there 
are  the  ctenophore,  the  flat-worm,  the  echino- 
derm,  the  annelid-mollusk  and  the  chordate 
types  of  cytoplasmic  localization  (Fig.  43). 
The  polarity,  symmetry  and  pattern  of  a 


THE   CELLULAR  BASIS 


173 


FIG.  41.  NORMAL  AND  INVERSE  SYMMETRY  OF  THE  3n,  4rH, 
STH  AND  GTH  CLEAVAGES.  The  cells  la-Id,  2a-2d  and  3a-3d  give 
rise  to  all  the  ectoderm;  4d  or  M  gives  rise  to  mesoderm; 
A,  B,  C,  D  to  endoderm. 


Fie.  42.     For  description   see   p.   175. 


THE   CELLULAR   BASIS  175 

jellyfish,  starfish,  worm,  mollusk,  insect  or 
vertebrate  are  foreshadowed  by  the  character- 
istic polarity,  symmetry  and  pattern  of  the 
cytoplasm  of  the  egg  either  before  or  immedi- 
ately after  fertilization.  In  all  of  these  phyla 
eggs  may  develop  without  fertilization,  either 
by  natural  or  by  artificial  parthenogenesis, 
and  in  such  cases  the  characteristic  polarity, 
symmetry  and  pattern  of  the  adult  are  found 
in  the  cytoplasm  of  the  egg  just  as  if  the  lat- 
ter had  been  fertilized.  The  conclusion  seems 
to  be  justified  that  these  earliest  and  most 
fundamental  differentiations  which  distin- 
guished the  eggs  of  various  phyla  are  not  de- 
pendent upon  the  sperm. 

All  of  these  correspondences  between  the 
polarity,  symmetry  and  pattern  of  the  egg 
and  of  the  developed  animal  are  found  in  the 
cytoplasm.  It  is  possible  that  the  polarity 


FIG.  42.  NORMAL  AND  INVERSE  SYMMETRY  IK  LATE  EM- 
BRYOS AND  ADULT  STAGES.  In  1,  cross-hatched  area  is  blasto- 
pore;  cells  shaded  by  lines,  mesoderm,  other  cells,  endoderm; 
the  spiral  twist  of  the  snail  begins  in  opposite  directions  in 
the  two  embryos.  In  2,  the  adult  organization  is  shown  with 
all  organs  inversely  symmetrical;  os,  olfactory  organ;  a,  anus; 
L,  lung;  V,  ventricle;  K,  kidney.  In  3,  sinistral  and  dextral 
shells  of  adult  snails  are  shown. 


176  HEREDITY  AND  ENVIRONMENT 

may  be  carried  over  from  generation  to  gen- 
eration through  the  egg  cell,  but  the  symmetry 
and  localization  pattern  develop  in  the  ovum 
before  or  just  after  maturation.  In  this  dif- 
ferentiation and  localization  of  the  egg  cyto- 
plasm it  is  probable  that  certain  influences 
have  come  from  the  nucleus  of  the  egg,  and 
perhaps  from  the  egg  chromosomes.  There  is 
no  doubt  that  most  of  the  differentiations  of 
the  egg  cytoplasm  have  arisen  during  the 
ovarian  history  of  the  egg,  and  as  a  result  of 
the  interaction  of  nucleus  and  cytoplasm;  but 
the  fact  remains  that  at  the  time  of  fertiliza- 
tion the  hereditary  potencies  of  the  two  germ 
cells  are  not  equal,  all  the  early  stages  of  de- 
velopment, including  the  polarity,  symmetry, 
type  of  cleavage,  and  the  pattern,  or  relative 
positions  and  proportions  of  future  organs, 
being  foreshadowed  in  the  cytoplasm  of  the 
egg  cell,  while  only  the  differentiations  of  later 
development  are  influenced  by  the  sperm.  In 
short  the  egg  cytoplasm  fixes  the  general  type 
of  development  and  the  sperm  and  egg  nuclei 
supply  only  the  details. 

We   are  vertebrates   because   our  mothers 


THE   CELLULAR  BASIS  177 

were  vertebrates  and  produced  eggs  of  the 
vertebrate  pattern;  but  the  color  of  our  skin 
and  hair  and  eyes,  our  sex,  stature,  and  mental 
peculiarities  were  determined  by  the  sperm  as 
well  as  by  the  egg  from  which  we  came.  There 
is  evidence  that  the  chromosomes  of  the  egg 
and  sperm  are  the  seat  of  the  differential  fac- 
tors or  determiners  for  Mendelian  characters, 
while  the  general  polarity,  symmetry  and  pat- 
tern of  the  embryo  are  determined  by  the  cyto- 
plasm of  the  egg  (see  p.  263). 

It  will  be  observed  that  the  correlation  be- 
tween chromosomes  and  adult  characters  is 
different  in  kind  from  that  between  the  cyto- 
plasm of  the  egg  and  the  adult  characters;  in 
the  latter  case  polarity,  symmetry  and  pat- 
tern are  of  the  same  kind  in  the  egg  and  in  the 
adult,  and  the  correspondence  is  comparatively 
close;  in  the  latter  there  is  no  correspondence 
in  kind  between  the  chromosomal  peculiarities 
and  the  pecularities  of  the  adult.  This  fact 
might  suggest  that  the  chromosomal  organiza- 
tion may  be  more  fundamental  than  that  of 
the  cytoplasm.  There  are  reasons  for  believ- 
ing that  many  substances  of  the  cell  are  formed 


CTENOPHORE 


TURBELLARIAN 


ASCIDIAN  II 


FIG.  43.  TYPES  OF  EGG  ORGANIZATION  IN  DIFFERENT  PHYLA; 
cross-hatched  area,  mesoderm  or  mesenchyme  (mes) ;  hori- 
zontal lines,  endoderm  (end);  clear  area,  ectoderm  (ect).  In 
the  first  four  figures  the  pattern  of  localization  is  that  which 
is  found  at  the  close  of  the  first  cleavage;  in  Ascidian  II  the 
pattern  is  that  which  is  found  at  the  close  of  the  second 
cleavage;  in  the  annelid  egg  the  localization  of  later  stages  is 
projected  upon  the  egg;  n.p.,  neural  plate;  ch.,  chorda; 
e.g.,  cerebral  ganglion;  v.g.,  ventral  ganglion;  proto., 
prototroch. 


THE   CELLULAR   BASIS  179 

by  the  interaction  of  nucleus  and  cytoplasm, 
and  most  probably  the  chromosomes  are  an 
important  factor  in  this  process.  But  in  no 
case  is  the  cytoplasm  a  neglible  factor,  in  no 
case  does  it  serve  merely  as  food  for  the 
chromosomes.  The  entire  cell,  nucleus  and 
cytoplasm,  is  concerned  in  heredity  and 
differentiation. 


D.  THE  MECHANISM  OF  DEVELOPMENT 

Development  consists  in  the  transformation 
of  the  oosperm  into  the  adult.  What  is  the 
mechanism  by  which  this  transformation  is 
effected?  There  is  progressive  differentiation 
of  the  germ  into  the  developed  organism  but 
by  what  process  is  this  differentiation  ac- 
complished? 

Many  different  processes  are  concerned  in 
embryonic  differentiation.  From  the  stand- 
point of  the  cell  the  most  important  of  these 
are  (1)  the  formation  of  different  kinds  of 
substances  in  cells,  (2)  the  localization  and 
isolation  of  these  substances,  (3)  the  trans- 
formation of  these  substances  into  the  various 


180  HEREDITY  AND  ENVIRONMENT 

structures  which  are  characteristic  of  the  dif- 
ferent kinds  of  tissue  cells.  We  shall  here  de- 
scribe only  the  first  and  second  of  these  pro- 
cesses which  are  of  more  general  interest  than 
the  last. 

1.  The  Formation  of  Different  Substances 
in  Cells. — Embryonic  differentiation  consists 
primarily  in  the  formation  of  different  kinds  of 
protoplasm  out  of  the  protoplasm  of  the  germ 
cells.  It  is  plain  that  different  kinds  of  proto- 
plasm are  present  in  the  two  germ  cells  before 
they  unite  in  fertilization,  but  in  the  course  of 
development  the  number  of  these  substances 
and  the  degree  of  difference  between  them 
greatly  increase. 

Actual  observation  shows  that  by  the  inter- 
action with  one  another  of  substances  or  parts 
originally  present  and  by  their  reactions  to 
external  stimuli  new  substances  and  parts  ap- 
pear which  had  no  previous  existence  just  as 
new  substances  result  from  chemical  reactions. 
This  is  "creative  synthesis"  in  philosophy, 
epigenesis  in  development.  Differentiations 
appear  chiefly  in  the  cytoplasm  but  only  as  the 
result  of  interaction  between  cytoplasm  and 


THE   CELLULAR   BASIS  181 

nucleus.  Similarly,  it  may  be  argued,  smaller 
units  of  organization  such  as  chromosomes  or 
chromomeres  do  not  in  themselves  give  rise  to 
any  adult  part,  but  only  as  they  interact  upon 
other  units  are  new  parts  formed. 

In  many  cases  the  first  formation  of  such 
new  substances  appears  in  the  immediate 
vicinity  of  the  nucleus  and,  like  assimilation 
itself,  is  evidently  brought  about  by  the  inter- 
action of  nucleus  and  cytoplasm.  In  certain 
cases  it  can  be  seen  that  the  achromatin  and 
oxychromatin  which  escape  from  the  nucleus 
during  division  take  part  in  the  formation  of 
new  substances  in  the  cell  body,  and  since  the 
oxychromatin  is  derived  from  the  chromo- 
somes of  the  previous  cell  division,  it  is  prob- 
able that  the  chromosomes  are  a  factor  in  this 
process. 

Weismann  maintained  that  the  chromo- 
somes and  the  inheritance  units  contained  in 
them  undergo  differentiation  by  a  process  of 
disintegration  and  that  these  disintegrated 
units  escape  into  the  cell  body  and  there  pro- 
duce different  kinds  of  cytoplasm  in  different 
cells.  A  somewhat  similar  view  was  advanced 


182  HEREDITY    AND    ENVIRONMENT 

by  deVries  in  his  theory  of  intra-cellular  pan- 
genesis.  However,  as  we  have  seen  already, 
there  is  good  evidence  that  the  chromosomes 
do  not  undergo  progressive  differentiation  in 
the  course  of  development;  they  always  divide 
with  exact  equality,  and  even  in  highly  differ- 
entiated tissue  cells  their  number  and  form 
usually  remain  as  in  embryonic  cells. 

On  the  other  hand  the  cytoplasm  undergoes 
progressive  differentiation,  and  when  by  pres- 
sure or  centrifugal  force  such  differentiated 
cytoplasm  is  brought  into  relations  with 
strange  nuclei  the  differentiations  of  the  cyto- 
plasm are  not  always  altered  thereby,  thus 
showing  that  the  different  nuclei  are  essentially 
alike  and  that  differentiations  are  mainly  lim- 
ited to  the  cytoplasm.  Thus  the  differentiations 
of  cells  are  not  due  to  the  differentiations  of 
their  nuclei,  but  rather  the  reverse  is  true,  such 
differentiations  of  nuclei  as  occur  are  due  to 
differentiations  of  the  cytoplasm  in  which  they 
lie.  Nevertheless  differentiations  do  not  take 
place  in  the  absence  of  nuclear  material,  and  it 
seems  probable  that  the  interaction  of  nucleus 
and  cytoplasm  is  necessary  to  the  formation  of 


THE   CELLULAR   BASIS  183 

the  new  cytoplasmic  substances  which  appear 
in  the  course  of  development. 

2.  Segregation  and  Isolation  of  Different 
Substances  in  Cells. — But  differentiation  con- 
sists not  only  in  the  formation  of  different 
kinds  of  substances  in  cells  but  also  in  the  sep- 
aration of  these  substances  from  one  another. 
This  separation  is  brought  about  to  a  great 
extent  by  flowing  movements  within  cells 
which  are  associated  especially  with  cell 
division. 

In  all  these  processes  of  heredity  and  de- 
velopment cell  division  plays  a  particularly 
important  part.  If  cell  divisions  were  always 
exactly  alike  there  could  be  no  initial  differ- 
ence between  the  daughter  cells,  and  unless 
acted  upon  by  different  stimuli  all  cells  would 
remain  exactly  alike.  But  there  is  much  evi- 
dence that  daughter  cells  are  often  unlike  from 
the  time  of  their  formation,  and  that  different 
stimuli  act  upon  them  to  increase  still  further 
this  initial  difference. 

(a)  Differential  and  Non-differential  Cell 
Division. — When  each  half  of  any  dividing 
unit  is  like  the  other  half  the  division  is  non- 


184  HEREDITY    AND    ENVIRONMENT 

differential.  So  far  as  we  know  the  di- 
visions of  all  the  smallest  elements  of  the  cell 
are  of  this  sort ;  there  is  no  good  evidence  that 
the  plastosomes,  the  chromomeres,  or  the  chro- 
mosomes ever  divide  into  unlike  halves,  though 
in  the  maturation  divisions  the  separation  of 
whole  chromosomes  leads  to  the  appearance  of 
a  differential  division  of  the  chromosomes. 
But  while  all  of  the  cell  elements  may  be  sup- 
posed to  grow  and  divide  into  equivalent 
halves  there  may  be  an  unequal  distribution 
of  these  halves  in  cell  division,  so  that  the  two 
daughter  cells  are  unlike.  This  is  what  is 
known  as  differential  cell  division  and  it  plays 
a  most  important  part  in  differentiation. 
While  the  chromosomes  are  equally  distributed 
to  the  daughter  cells,  except  in  the  case  of  the 
maturation  divisions,  the  achromatin  and  the 
oxychromatin  of  the  nucleus  are  not  always 
distributed  equally  and  this  is  probably  an 
important  factor  in  development.  The  divi- 
sions of  the  cytoplasm  of  the  egg  are  fre- 
quently differential  and  such  divisions  are 
known  to  play  a  great  part  in  the  embryonic 
differentiation. 


THE   CELLULAR   BASIS  185 

In  the  differential  divisions  of  the  cytoplasm 
unlike  substances  become  localized  in  certain 
parts  of  the  cell  body,  chiefly  by  means  of 
definite  flowing  movements  of  the  cytoplasm, 
and  when  cell  division  occurs  these  substances 
become  permanently  separated  by  partition 
walls.  In  this  way  irreversible  differentiations 
are  formed.  If  the  formation  of  partition 
walls  is  prevented  the  different  substances 
within  the  cell  body  may  freely  commingle,  es- 
pecially during  nuclear  division  when  the  cyto- 
plasmic  movements  are  especially  active;  in 
such  cases  differentiation  may  be  arrested  even 
though  nuclear  division  continues.  In  the  de- 
veloping eggs  of  most  animals  partition  walls 
between  daughter  cells  are  necessary  to  pre- 
vent the  commingling  of  different  kinds  of 
substances,  which  are  sorted  by  the  movements 
within  the  cell  and  are  isolated  by  the  partition 
walls.  In  some  cases,  as  for  example  in  cer- 
tain protozoa,  the  commingling  of  different 
kinds  of  protoplasm  within  a  cell  may  be  pre- 
vented by  the  viscosity  of  portions  of  the  pro- 
toplasm, or  by  the  formation  of  intracellular 
membranes,  or  by  a  reduction  to  a  minimum  of 


186  HEREDITY    AND    ENVIRONMENT 

the  mitotic  movements  within  the  cell  by  the 
persistence  of  the  nuclear  membrane  during 
division.  In  general  the  degree  of  differentia- 
tion may  be  measured  by  the  degree  of  un- 
likeness  between  different  cells,  and  by  the 
completeness  with  which  the  protoplasm  of 
different  cells  is  kept  from  intermingling. 

SUMMARY 

All  the  phenomena  of  life,  including  hered- 
ity and  development,  are  cellular  phenomena 
in  that  they  include  only  the  activities  of 
cells  or  of  cell  aggregates.  The  cell  is  the 
ultimate  independent  unit  of  organic  structure 
and  function.  The  only  living  bond  between 
one  generation  and  the  next  is  found  in  the 
sex  cells  and  all  inheritance  must  take  place 
through  these  cells.  Inherited  traits  are  not 
transmitted  from  parents  to  offspring  but  the 
germinal  factors  or  causes  are  transmitted, 
and  under  proper  conditions  of  environment 
these  give  rise  to  developed  characters.  Every 
oosperm  as  well  as  every  developed  organism 
differs  more  or  less  from  every  other  one  and 
this  remarkable  condition  is  brought  about  by 


THE   CELLULAR   BASIS  187 

extremely  numerous  permutations  in  the  dis- 
tribution of  certain  parts  of  the  sex  cells  in 
maturation  and  fertilization.  Sex  is  an  in- 
herited character  dependent,  probably,  upon 
an  alternative  distribution  of  certain  chromo- 
somes of  the  nucleus.  There  is  much  evidence 
that  the  factors  for  all  sorts  of  alternative 
characters  are  associated  with  the  chrom- 
osomes. The  differentiation  of  the  oosperm 
into  the  developed  organism  is  accomplished 
in  part  by  the  associations  and  dissociations  of 
germinal  units  which  lead  to  the  formation  of 
new  materials,  and  in  part  by  the  segregation 
and  localization  of  these  in  definite  cells. 

Germ  cells  and  probably  all  other  kinds  of 
cells  are  almost  incredibly  complex.  We  know 
that  former  students  of  the  cell  greatly  under- 
estimated this  complexity  and  there  is  no 
reason  to  suppose  that  we  have  fully  compre- 
hended it.  What  Darwin  said  of  the  entire 
organism  may  now  be  said  of  every  cell:  "An 
organic  being  is  a  microcosm — a  little  universe, 
formed  of  a  host  of  self -propagating  organ- 
isms, inconceivably  minute  and  numerous  as 
the  stars  in  heaven." 


CHAPTER  III 
PHENOMENA  OF  INHERITANCE 


CHAPTER  III 

PHENOMENA  OF  INHERITANCE 
A.  OBSERVATIONS  ON  INHERITANCE 

The  observations  of  men  in  all  ages  have 
established  the  fact  that  in  general  "like  pro- 
duces like,"  and  that,  in  spite  of  many  excep- 
tions, children  are  in  their  main  characteristics 
like  their  parents.  And  yet  offspring  are 
never  exactly  like  their  parents,  and  this  has 
led  to  the  saying  that  "like  does  not  produce 
like  but  only  somewhat  like."  What  is  meant 
is  that  there  are  general  resemblances  but 
particular  differences  between  parents  and 
offspring. 

INDIVIDUALS   AND   THEIR   CHARACTERS 

In  considering  organic  individuals  one  may 
think  of  them  as  wholes  or  as  composed  of 
parts,  as  indivisible  unities  or  as  constituent 

191 


192  HEREDITY    AND    ENVIRONMENT 

characters;  either  aspect  is  a  true  one  and 
yet  neither  is  complete  in  itself.  Formerly  in 
discussions  on  heredity  the  individual  was  re- 
garded in  its  entirety  and  when  all  hereditary 
resemblances  and  differences  were  averaged  it 
was  said  that  one  child  resembled  the  father, 
another  child  the  mother.  This  method  of 
lumping  together  and  averaging  resemblances 
and  differences  led  to  endless  confusion.  In 
heredity  no  less  than  in  anatomy  it  is  neces- 
sary to  deal  with  the  constituents  of  organ- 
isms; in  short,  the  organism  must  be  analyzed 
and  each  part  studied  by  itself.  Francis  Gal- 
ton  was  one  of  the  first  to  bring  order  out  of 
chaos  by  dealing  with  traits  or  characters 
singly  instead  of  treating  all  together.  He 
made  careful  studies  on  the  inheritance  of 
weight  and  size  in  the  seeds  of  sweet  peas,  and 
on  the  inheritance  of  stature,  eye-color,  intel- 
lectual capacity,  artistic  ability  and  certain 
diseases  in  man.  At  the  same  time  that  Gal- 
ton  was  thus  laying  the  foundations  for  a 
scientific  study  of  heredity  by  dealing  with 
characters  separately,  another  and  even 
greater  student  of  heredity,  Gregor  Mendel, 


PHENOMENA  OF  INHERITANCE  193 

was  doing  the  same  thing  in  his  experiments 
with  garden  peas,  but  inasmuch  as  Mendel's 
work  remained  practically  unknown  for  many 
years,  Galton  has  been  rightly  recognized  as 
the  founder  of  the  scientific  study  of  heredity. 
Of  course,  neither  Galton  nor  anyone  else 
who  has  followed  his  method  of  dealing  with 
the  characters  of  organisms  singly,  ever  sup- 
posed that  such  characters  could  exist  inde- 
pendently of  other  characters  and  apart  from 
the  entire  organism.  This  is  such  a  self-evi- 
dent fact  that  it  may  seem  needless  to  mention 
it,  and  yet  there  have  been  critics  who  have 
believed,  or  have  assumed  to  believe,  that 
modern  students  of  heredity  attempt  to  an- 
alyze organisms  into  independently  existing 
characters,  whereas  in  most  cases  they  have 
done  only  what  the  anatomist  does  in  treating 
separately  the  various  organs  of  the  body. 

HEREDITARY    RESEMBLANCES    AND    DIFFERENCES 

The  various  characters  into  which  an  organ- 
ism may  be  analyzed  show  a  greater  or  smaller 
degree  of  resemblance  to  the  corresponding 
characters  of  its  parents.  Whenever  the  dif- 


194  HEREDITY    AND    ENVIRONMENT 

ferential  cause  of  a  character  is  a  germinal 
one  the  character  is,  by  definition,  inherited; 
on  the  other  hand,  whenever  this  differential 
cause  is  environmental  the  character  is  not 
inherited.  While  it  is  true  that  inheritance  is 
most  clearly  recognized  in  those  characters  in 
which  offspring  resemble  their  parents,  even 
characters  in  which  they  differ  from  their 
parents  may  be  inherited,  as  is  plainly  seen 
when,  in  any  character,  a  child  resembles  a 
grandparent  or  a  more  distant  ancestor  more 
than  either  parent.  Sometimes  actually  new 
characters  arise  in  descendants  which  were 
not  present  in  ascendants,  but  which  are  there- 
after inherited.  Accordingly  inherited  char- 
acters may  be  classified  as  resemblances  and 
differences,  though  both  are  determined  by 
germinal  organization,  or  heredity.  There  is 
therefore  no  fundamental  difference  between 
inherited  similarities  and  dissimilarities.  Her- 
edity and  variation  are  not  opposing  nor  con- 
trasting tendencies  which  make  offspring  like 
their  parents  in  one  case  and  unlike  them  in 
another;  really  inherited  characters  may  be 
like  or  unlike  those  of  the  parents. 


PHENOMENA  OF  INHERITANCE  195 

On  the  other  hand  many  resemblances  and 
differences  between  parents  and  offspring  are 
due  not  to  heredity  at  all,  but  to  environ- 
mental conditions.  By  means  of  experiment 
it  is  possible  to  distinguish  between  hereditary 
and  environmental  resemblances  and  differ- 
ences, but  among  men  where  experiments  are 
generally  out  of  the  question  it  is  often  diffi- 
cult or  impossible  to  make  this  distinction. 

I.  HEREDITARY  RESEMBLANCES 

1.  Racial  Characters. — All  peculiarities 
which  are  characteristic  of  a  race,  species, 
genus,  order,  class  and  phylum  are  of  course 
inherited,  otherwise  there  would  be  no  con- 
stant characteristics  of  these  groups  and  no 
possibility  of  classifying  organisms.  The  chief 
characters  of  every  living  thing  are  unalter- 
ably fixed  by  heredity.  Men  do  not  gather 
grapes  of  thorns  nor  figs  of  thistles.  Every 
living  thing  produces  offspring  after  its  own 
kind.  Men,  horses,  cattle;  birds,  reptiles, 
fishes;  insects,  mollusks,  worms;  polyps, 
sponges,  micro-organisms, — all  of  the  million 


196  HEREDITY    AND    ENVIRONMENT 

known  species  of  animals  and  plants  differ 
from  one  another  because  of  inherited  peculi- 
arities, because  they  have  come  from  differ- 
ent kinds  of  germ  cells. 

2.  Individual  Characters. — Many  charac- 
ters which  are  peculiar  to  certain  individuals 
are  known  to  be  inherited,  and  in  general  use 
the  word  "inheritance"  refers  to  the  repetition 
in  successive  generations  of  such  individual 
peculiarities.  Among  such  individual  charac- 
ters are  the  following: 

(a)  Morphological  Features. — Hereditary 
resemblances  are  especially  recognizable  in  the 
gross  and  minute  anatomy  of  every  organism 
in  the  form,  structure,  location,  size,  color,  etc., 
of  each  and  every  part.  The  number  of  such 
individual  peculiarities  which  are  inherited  is 
innumerable  and  only  a  few  of  the  more  strik- 
ing of  these  can  be  mentioned. 

It  is  a  matter  of  common  knowledge  that 
unusually  great  or  small  stature  runs  in  cer- 
tain families,  and  Galton  developed  a  formula 
for  determining  the  approximate  stature  of 
children  from  the  known  stature  of  the  parents 
and  from' the  mean  stature  of  the  race.  How- 


PHENOMENA  OF  INHERITANCE  197 

ever,  his  statistical  and  mathematical  formula 
give  only  general  or  average  results,  from 
which  there  are  many  individual  departures 
and  exceptions. 

In  the  same  way  the  color  of  the  skin,  the 
color  and  form  of  hair  and  the  color  of  eyes 
are  in  general  like  those  of  one  or  more  of 
the  parents  or  grandparents.  We  all  know 
that  certain  facial  features  such  as  the  shape 
and  size  of  eyes,  nose,  mouth  and  chin  are 
generally  characteristic  of  certain  families. 

But  the  inheritance  of  anatomical  features 
extends  to  much  more  minute  characters  than 
those  just  mentioned.  In  certain  families  a 
few  hairs  in  the  eyebrows  are  longer  than  the 
others,  or  there  may  be  patches  of  parti- 
colored hair  over  the  scalp,  or  dimples  in  the 
cheek,  chin,  or  other  parts  of  the  skin  may  oc- 
cur, and  these  trifling  peculiarities  are  in- 
herited with  all  the  tenacity  shown  in  the 
transmission  of  more  important  characters, 
Johannsen  has  found  races  of  beans  in  which 
the  average  weight  of  individual  seeds  differed 
only  by  .02  to  .03  gram,  and  yet  these  minute 
differences  in  weight  were  characteristic  of 


198  HEREDITY    AND    ENVIRONMENT 

each  race  and  were  of  course  inherited.  Jen- 
nings has  found  races  of  Paramedum  which 
show  hereditary  differences  of  .005  mm.  in 
length  (Fig.  44).  Nettleship  says  that  the 


FIG.  44.  Diagram  of  eight  different  races  of  Paramedum, 
each  horizontal  row  (A — H)  representing  a  single  race.  The 
individual  showing  the  mean  size  in  each  race  is  indicated  by 
-j-  ;  the  mean  of  all  the  races  is  shown  by  the  line  X  —  X. 
The  numbers  are  the  lengths  in  micra  (-thousandths  of 
a  millimeter),  X  *3-  (After  Jennings.) 


PHENOMENA  OF  INHERITANCE  199 

lens  of  the  human  eye  weighs  only  about  175 
milligrams,  or  about  one  three-millionth  part 
of  the  body  weight,  and  in  hereditary  cataract 
only  about  one  twentieth  part  of  the  lens  be- 
comes opaque,  and  yet  this  minute  fraction  of 
the  body  weight  shows  the  influence  of  hered- 
ity. Even  the  size,  shape  and  number  of  the 
cells  in  certain  organs,  and  in  given  embry- 
onic stages,  may  be  repeated  generation  after 
generation;  and  if  our  analysis  were  suffi- 
ciently complete  we  should  doubtless  find  that 
even  the  minute  parts  of  cells,  such  as  nuclei, 
chromosomes  and  centrosomes,  show  individ- 
ual peculiarities  which  are  inherited. 

(b)  Teratological  and  Pathological  Peculi- 
arities are  really  only  unusual  or  abnormal  an- 
atomical characters,  but  of  such  interest  and 
importance  as  to  deserve  special  mention, 
Many  such  abnormalities  are  undoubtedly  in- 
herited, among  which  are  the  following:  poly- 
dactylism,  in  which  more  than  the  normal 
number  of  digits  are  present;  syndactylism. 
or  a  condition  of  webbed  fingers  and  toes; 
brachydactylism,  in  which  the  fingers  are  short 
and  stumpy  and  usually  contain  less  than  the 


200  HEREDITY    AND    ENVIRONMENT 

normal  number  of  joints;  acondroplasy,  or 
short  and  crooked  limbs,  such  as  occur  in  cer- 
tain breeds  of  dogs  and  sheep  and  in  certain 
human  dwarfs ;  myopia,  in  which  the  eyeball  is 
elongated;  glaucoma,  or  swelling  of  the  eye- 
ball; coloboma,  or  open  suture  of  the  iris; 
otosclerosis,  or  thickened  tympanic  mem- 
brane, causing  "hardness  of  hearing";  some 
forms  of  deaf-mutism,  due  to  certain  defects 
of  the  inner  ear;  and  many  other  characters 
too  numerous  to  mention  here.  On  the  other 
hand  many  abnormal  or  monstrous  conditions 
are  due  to  abnormal  environment  and  are  not 
inherited. 

The  question  of  the  inheritance  of  diseases 
may  be  briefly -considered  here.  If  a  disease  is 
due  to  some  defect  in  the  hereditary  constitu- 
tion, it  is  inherited;  otherwise,  according  to 
our  definition  of  heredity,  it  is  not.  Of  course 
no  disease  develops  without  extrinsic  causes 
but  when  one  individual  takes  a  disease  while 
another  under  the  same  conditions  does  not, 
the  differential  cause  may  be  an  inherited  one, 
or  it  may  be  due  to  differences  in  the  previous 
conditions  of  life.  There  is  no  doubt  thai 


PHENOMENA  OF  INHERITANCE  201 

certain  diseases  run  in  families  and  have  the 
appearance  of  being  inherited,  but  in  this  case 
as  in  many  others  it  is  extremely  difficult  in 
the  absence  of  experiments  to  distinguish  be- 
tween effects  due  to  intrinsic  causes  and  those 
due  to  extrinsic  ones.  Where  the  specific 
cause  of  a  disease  is  some  microorganism  the 
individual  must  have  been  infected  at  some 
time  or  other,  almost  invariably  after  birth. 
In  few  instances  is  the  oosperm  itself  infected, 
and  even  when  it  is,  this  is  not,  strictly  speak- 
ing, a  case  of  inheritance,  but  rather  one  of 
early  infection.  Pearson  has  found  that  there 
is  a  marked  correlation  (represented  by  the 
number  .55  when  complete  correlation  is  1.) 
between  tuberculous  parents  and  tubercu- 
lous children,  but  there  is  very  little  evidence 
that  the  child  is  ever  infected  before  birth. 
What  is  inherited  in  this  case  is  probably 
slight  resistance  to  the  tubercle  bacillus.  There 
is  evidence  that  almost  all  adult  persons  have 
been  infected  at  one  time  or  another  by  this 
bacillus,  but  it  has  not  developed  far  in  all  of 
them  because  some  have  superior  powers  of 
resistance.  Such  greater  or  smaller  resistance, 


202  HEREDITY    AND    ENVIRONMENT 

stronger  or  weaker  build,  is  inherited,  and 
while  diminished  resistance  is  not  the  direct 
cause  of  tuberculosis  it  is  a  predisposing  cause. 
The  same  is  probably  true  of  many  other 
diseases,  the  immediate  causes  of  which  are 
extrinsic,  while  only  the  more  remote,  or  pre- 
disposing causes,  are  hereditary. 

(c)  Physiological  peculiarities  are  inherited 
as  well  as  morphological  ones ;  indeed  function 
and  structure  are  only  two  aspects  of  one  and 
the  same  thing,  namely  organization.  For  all 
morphological  characters  there  are  functional 
correlatives,  for  functional  characters  morpho- 
logical expressions,  and  if  the  one  is  inherited 
so  is  the  other.  But  there  are  certain  char- 
acters in  which  the  physiological  aspect  is 
more  striking  than  the  morphological  one. 

For  example,  longevity  is  a  physiological 
character  which  is  undoubtedly  dependent 
upon  many  causes,  but  in  the  case  of  species 
which  differ  greatly  in  length  of  life  there  can 
be  little  doubt  that  we  are  dealing  with  an  in- 
herited character.  The  great  differences  in 
the  length  of  life  of  an  elephant  and  a  mouse, 
of  a  parrot  and  a  pigeon,  of  a  cicada  and  a 


PHENOMENA  OF  INHERITANCE  203 

squash  bug,  are  as  surely  the  result  of  in- 
herited causes  as  are  the  structural  differences 
between  those  animals.  Within  the  same 
species  different  races  or  lines  show  character- 
istic differences  in  length  of  life;  in  the  case 
of  man  the  average  length  of  life  is  much 
greater  in  some  families  than  in  others,  and 
life  insurance  companies  take  account  of  this 
fact.  Even  within  the  same  organism  certain 
organs  or  cells  are  short-lived,  whereas  others 
are  long-lived;  some  cells  and  organs  live  only 
through  the  early  embryonic  period,  while 
others  live  as  long  as  the  general  organism. 

Obesity  is  another  physiological  character- 
istic which  may  be  inherited;  the  members  of 
certain  families  grow  fat  in  spite  of  themselves, 
while  members  of  other  families  remain  thin 
however  well  fed  they  may  be.  Here  also 
many  factors  enter  into  the  result,  but  it  seems 
probable  that  the  differentiating  factor  is  an 
hereditary  one.  Baldness  affects  the  male 
members  of  certain  families  when  they  have 
reached  a  given  age,  while  in  others  neither 
care,  dissipation  nor  age  can  rob  a  man  of  his 
bushy  top.  Haemophilia,  or  excessive  bleed- 


204  HEREDITY   AND   ENVIRONMENT 

ing  after  an  injury,  which  is  due  to  a  defi- 
ciency in  the  clotting  power  of  the  blood,  is 
strongly  inherited  in  the  male  line  in  certain 
families.  Fecundity  and  a  tendency  to  bear 
twins  or  triplets,  left-handedness,  a  peculiar 
lack  of  resistance  to  certain  diseases,  and  many 
other  physiological  peculiarities  are  probably 
inherited. 

(d)  Psychological  characters  appear  to  be 
inherited  in  the  same  way  that  anatomical  and 
physiological  traits  are;  indeed  all  that  has 
been  said  regarding  the  correlation  of  morpho- 
logical and  physiological  characters  applies 
also  to  psychological  ones.  No  one  doubts 
that  particular  instincts,  aptitudes  and  ca- 
pacities are  inherited  among  both  animals  and 
men,  nor  that  different  races  and  species  differ 
hereditarily  in  psychological  characteristics. 

Certain  breeds  of  dogs  such  as  the  mastiff, 
the  bull  dog,  the  terrier,  the  collie  and  many 
others  are  characterized  by  peculiarities  of 
temperament,  affection,  intelligence  and  dis- 
position. No  one  who  has  much  studied  the 
subject  can  doubt  that  different  human  races 
and  families  show  characteristic  differences  in 


PHENOMENA  OF  INHERITANCE  205 

these  same  respects.  It  is  quite  futile  to  argue 
that  exceptional  individuals  may  be  found  in 
one  race  with  the  mental  characteristics  of  an- 
other race ;  the  same  could  be  said  of  different 
breeds  of  dogs,  or  of  the  sizes  of  different  races 
of  beans  or  of  Paramecia  (Fig.  44) .  The  fact 
is  that  racial  characteristics  are  not  determined 
by  exceptional  and  extreme  individuals  but  by 
the  average  or  mean  qualities  of  the  race;  and 
measured  in  this  way  there  is  no  doubt  that 
certain  types  of  mind  and  disposition  are  char- 
acteristic of  certain  families. 

There  is  no  longer  any  question  that  some 
kinds  of  feeble-mindedness,  epilepsy  and  in- 
sanity are  inherited,  and  that  there  is  often  an 
hereditary  basis  for  nervous  and  phlegmatic 
temperaments,  for  emotional,  judicial  and 
calculating  dispositions.  Nor  can  it  be  de- 
nied that  strength  or  weakness  of  will,  a  ten- 
dency to  moral  obliquity  or  rectitude,  capacity 
or  incapacity  for  the  highest  intellectual  pur- 
suits, occur  frequently  in  certain  families  and 
appear  to  be  inherited.  In  spite  of  certain 
noteworthy  exceptions,  which  may  perhaps  be 
due  to  remarkable  variations,  statistics  col- 


206  HEREDITY    AND    ENVIRONMENT 

lected  by  Galton  show  that  genius  runs  in  cer- 
tain families ;  while  the  work  of  certain  recent 
investigators,  particularly  Goddard,  Daven- 
port and  Weeks,  proves  that  feeble-mindedness 
and  epilepsy  are  also  inherited ;  and  the  careful 
work  of  Mott  and  of  Rosanoff  leaves  no  room 
for  doubt  that  certain  types  of  insanity  are 
hereditary.  It  frequently  happens  that  fami- 
lies in  which  hereditary  insanity  occurs  also 
have  other  members  afflicted  with  epilepsy, 
hysteria,  alcoholism,  etc.,  which  would  indicate 
that  the  thing  inherited  is  an  unstable  condi- 
tion of  the  nervous  system  which  may  take 
various  forms  under  slightly  different  condi- 
tions. Indeed  there  is  a  good  deal  of  evidence 
that  extraordinary  ability,  or  genius,  is  fre- 
quently associated  with  an  unstable  nervous 
organization  which  may  sometimes  take  the 
form  of  insanity  or  epilepsy  or  alcoholism. 
There  is  perhaps  more  truth  than  poetry  in 
Dryden's  lines: 

"Great  wits  are  sure  to  madness  near  allied, 

And  thin  partitions  do  their  bounds  divide." 
Woods  has  collected  data  concerning  "Hered- 
ity  in   Royalty"   which   seem   to   show   that 


PHENOMENA  OF  INHERITANCE  207 

very  high  or  low  grades  of  intellect  and 
virtues  may  be  traced  through  the  royal  fami- 
lies of  Europe  for  several  generations. 

The  general  tendency  of  recent  work  on 
heredity  is  unmistakable,  whether  it  concerns 
man  or  lower  animals.  The  entire  organism, 
consisting  of  structures  and  functions,  body 
and  mind,  develops  out  of  the  germ,  and  the 
organization  of  the  germ  determines  all  the 
possibilities  of  development  of  the  mind  no 
less  than  of  the  body,  though  the  actual  reali- 
zation of  any  possibility  is  dependent  also  upon 
environmental  stimuli. 

II.  HEREDITARY  DIFFERENCES 
There  are  many  limitations  or  exceptions 
to  the  general  rule  that  children  resemble  their 
parents.  Sometimes  these  differences  are  due 
to  new  combinations  of  ancestral  characters, 
sometimes  they  are  actually  new  characters 
not  present  so  far  as  known  in  any  of  the  an- 
cestors, though  even  such  new  characters  must 
arise  from  new  combinations  of  the  elements 
of  old  characters,  as  we  shall  see  later. 

1.  New  Combinations  of  Characters. — In 
all  cases  of  sexually  produced  organisms  new 


208  HEREDITY    AND    ENVIRONMENT 

combinations  of  ancestral  characters  are  evi- 
dent. Usually  a  child  inherits  some  traits 
from  one  parent  and  other  traits  from  the 
other  parent,  so  that  it  is  a  kind  of  mosaic  of 
ancestral  traits.  Such  inheritance,  bit  by  bit, 
of  this  character  from  one  progenitor  and  that 
from  another  was  described  by  Galton  as  "par- 
ticulate"  (Fig.  45).  On  the  other  hand  Galton 
supposed  that  in  some  instances  a  child  might 
inherit  all  or  nearly  all  of  his  traits  from  one 
parent,  a  thing  which  is  most  improbable ;  such 
inheritance  he  called  "alternative"  (Fig.  45). 

BLENDING  ALTERNATIVE  PARTICULATE 


O 


FIG.  45.     Diagram   to   illustrate   three   kinds   of   inheritance 
described  by  Galton.      (After  Walter.) 


PHENOMENA  OF  INHERITANCE  209 

In  other  cases  the  traits  of  the  parents  ap- 
pear to  blend  in  the  offspring,  as  for  example, 
in  the  skin  color  of  mulattoes ;  such  cases  were 
called  by  Galton  "blending"  inheritance  (Fig. 
45).  Sometimes  characters  appear  in  off- 
spring which  were  "latent"  in  the  parents  but 
were  "patent"  in  one  or  more  of  the  grand- 
parents ;  such  skipping  of  a  generation,  during 
which  a  character  remains  "latent,"  has  long 
been  known  as  "atavism."  At  other  times 
characters  which  were  present  in  distant  ances- 
tors, but  which  have  since  dropped  out  of 
sight  or  have  remained  "latent,"  reappear  in 
descendants;  such  cases  are  known  as 
"reversions." 

In  still  other  cases  certain  characters  ap- 
pear only  in  the  male  sex,  others  only  in  the 
female,  this  being  called  "sex-limited"  inheri- 
tance; while  in  some  instances  characters  are 
transmitted  from  fathers  through  daughters 
to  grandsons  or  from  mothers  to  sons,  all  such 
cases  being  known  as  "sex-linked"  inheritance. 

2.  New  Characters  or  Mutations. — But  in 
addition  to  these  permutations  in  the  distri- 
bution and  combination  of  ancestral  characters 


210  HEREDITY    AND    ENVIRONMENT 

new  and  unexpected  characters  sometimes  de- 
velop in  the  offspring,  which  were  not  present, 
so  far  as  known,  in  any  of  the  ascendants,  but 
which,  after  they  have  once  appeared,  are 
passed  on  by  heredity  to  descendants.  Such 
inherited  variations  are  usually  of  two  kinds, 
continuous  or  slight,  and  discontinuous  or 
sudden  variations.  The  latter  are  especially 
noticeable  when  variations  occur  in  the  normal 
number  of  parts,  as  in  four-leaved  clover,  or 
six-fingered  men,  and  such  numerical  varia- 
tions have  been  called  by  Bateson  "meristic." 
However,  sudden  variations  may  include  any 
marked  departure  from  the  normal  type,  in 
color,  shape,  size,  chemical  compositions,  etc. 
Such  sudden  variations  have  long  been  known 
to  breeders  as  "sports,"  and  both  Darwin  and 
Galton  pointed  out  the  fact  that  such  sports 
have  sometimes  given  rise  to  new  races  or 
breeds,  though  Darwin  was  not  inclined  to 
assign  much  importance  to  them  in  the  gen- 
eral process  of  evolution.  Galton,  on  the  other 
hand,  maintained  that  variations,  or  what 
would  now  be  called  "continuous  variations," 
cannot  be  of  much  significance  in  the  process 


PHENOMENA  OF  INHERITANCE  211 

of  evolution,  but  that  the  case  is  quite  different 
with  "sports." 

More  recently  the  entire  biological  world 
has  been  greatly  influenced  by  the  "Mutation 
Theory"  of  deVries,  which  has  placed  a  new 
emphasis  upon  the  importance  of  sudden  vari- 
ations in  the  process  of  evolution.  At  first 
deVries  was  inclined  to  emphasize  the  degree 
of  difference  between  discontinuous  and  con- 
tinuous variations,  but  in  later  works  this  dis- 
tinction is  given  a  minor  place  as  compared 
with  the  distinction  between  inherited  and 
non-inherited  variations.  Inherited  variations, 
whether  large  or  small,  are  called  by  deVries 
"mutations,"  whereas  non-inherited  variations 
are  known  as  "fluctuations."  The  former  are 
caused  by  changes  in  germinal  constitution, 
the  latter  by  alterations  in  environmental  con- 
ditions; the  former  represent  changes  in  he- 
redity, the  latter  changes  in  development. 

3.  Mutations  and  Fluctuations. — This  clear 
cut  distinction  between  mutations  and  fluctu- 
ations marks  one  of  the  most  important  ad- 
vances ever  made  in  the  study  of  development 
and  evolution.  Thousands  of  fluctuations  oc- 


212  HEREDITY  AND  ENVIRONMENT 

cur  which  are  purely  somatic  in  character  and 
which  do  not  affect  the  germ  cells,  for  every 
single  mutation  or  change  in  the  hereditary 
constitution;  and  yet  only  the  latter  are  of 
significance  in  heredity  and  evolution.  This 
distinction  between  variations  due  to  environ- 
ment (fluctuations)  and  those  due  to  hered- 
itary causes  (mutations)  was  recognized  by 
Weismann  and  many  of  his  followers,  but  the 
actual  demonstration  on  a  large  scale  of  the 
importance  of  this  distinction  is  due  largely  to 
deVries. 

All  hereditary  variations,  whether  due  to 
new  combinations  of  old  characters  or  to 
the  appearance  of  actually  new  characters, 
whether  small  and  continuous  or  large  and 
discontinuous,  have  their  causes  in  the  organi- 
zation of  the  germ  cells,  just  as  do  inherited 
resemblances.  Heredity  is  not  to  be  con- 
trasted with  variation,  nor  are  hereditary  like- 
ness and  unlikeness  due  to  conflicting 
principles;  both  are  the  results  of  germinal 
organization  and  both  are  phenomena  of 
heredity. 

4..  Every  Individual  Unique. — As  a  result 


PHENOMENA  OF  INHERITANCE  213 

of  the  permutations  of  ancestral  characters, 
the  appearance  of  mutations,  and  the  fluctua- 
tions of  organisms  due  to  environmental 
changes,  it  happens  that  in  all  cases  offspring 
differ  more  or  less  from  their  parents  and  from 
one  another.  No  two  children  of  the  same 
family  are  ever  exactly  alike  (except  in  the 
case  of  identical  twins  which  have  come  from 
the  same  oosperm).  Every  living  being  ap- 
pears on  careful  examination  to  be  the  first 
and  last  of  its  identical  kind.  This  is  one  of 
the  most  remarkable  peculiarities  of  living 
things.  The  elements  of  chemistry  are  con- 
stant, and  even  the  compounds  fall  into 
definite  categories  which  have  constant  charac- 
teristics. But  the  individuals  of  biology  are 
apparently  never  twice  the  same.  This  may 
be  due  to  the  immense  complexity  of  living 
units  as  contrasted  with  chemical  ones,  indeed 
lack  of  constancy  is  evidence  in  itself  of  lack 
of  analysis  into  real  elements  or  of  lack  of 
uniform  conditions;  but  whatever  its  cause 
the  extraordinary  fact  remains  that  every  liv- 
ing being  appears  to  be  unique.  "Reproduc- 
tion is  the  generation  of  unique  beings  that 


214  HEREDITY    AND    ENVIRONMENT 

are,  on  the  average,  more  like  their  kind  than 
like  anything  else"  (Brooks). 

There  seems  to  be  no  reason  to  doubt  that 
all  the  extraordinary  differences  which  organ- 
isms show,  as  well  as  all  of  their  resemblances, 
are  due  to  differences  or  resemblances  in  the 
hereditary  and  environmental  factors  which 
have  been  operative  in  their  development.  But 
in  view  of  this  universal  variability  of  organ- 
isms it  is  not  surprising  that  inheritance  has 
seemed  capricious  and  uncertain, — "a  sort  of 
maze  in  which  science  loses  itself." 

B.  STATISTICAL  STUDY  OF  INHERITANCE 

Francis  Galton  was  one  of  the  first  who  at- 
tempted to  reduce  the  mass  of  conflicting  ob- 
servations on  heredity  and  variation  to  some 
system  and  to  establish  certain  principles  as  a 
result  of  statistical  study.  He  was  the  real 
founder  of  the  scientific  study  of  inheritance; 
he  studied  characters  singly  and  introduced 
quantitative  measures.  Galton's  researches, 
which  were  published  in  several  volumes,  con- 
sisted chiefly  in  a  study  of  certain  families 


PHENOMENA  OF  'INHERITANCE  215 

with  regard  to  several  selected  traits,  viz., 
genius  or  marked  intellectual  capacity,  artis- 
tic faculty,  stature,  eye  color  and  disease.  As 
a  result  of  his  very  extensive  studies  two  main 
principles  appeared  to  be  established: 

1.  The  Law  of  Ancestral  Inheritance  which 
he  stated  as  follows: 

The  two  parents  contribute  between  them  on  the 
average  one-half  of  each  inherited  faculty,  each  of 
them  contributing  one-quarter  of  it.  The  four 
grandparents  contribute  between  them  one-quarter, 
or  each  of  them  one-sixteenth;  and  so  on,  the  sum 
of  the  series  1/2  +  1/4  -f-  1/8  +  1/16  .  .  .  being 
equal  to  1,  as  it  should  be.  It  is  a  property  of  this 
infinite  series  that  each  term  is  equal  to  the  sum  of 
all  those  that  follow :  thus  1/2  =  1/4  +  1/8  +  1/16 
+  ...,1/4  =  1/8  +  1/16  +  .  .  .  ,  and  so  on. 
The  prepotencies  of  particular  ancestors  in  any 
given  pedigree  are  eliminated  by  a  law  which  deals 
only  with  average  contributions,  and  the  various 
prepotencies  of  sex  with  respect  to  different  quali- 
ties are  also  presumably  eliminated. 

The  average  contribution  of  each  ancestor 
was  thus  stated  definitely,  the  contribution  di- 
minishing with  the  remoteness  of  the  ancestor. 
This  Law  of  Ancestral  Inheritance  is  repre- 
sented graphically  in  the  accompanying  dia- 


216 


HEREDITY    AND    ENVIRONMENT 


gram  (Fig.  46).  Pearson  has  somewhat 
modified  the  figures  given  by  Galton,  holding 
that  in  horses  and  dogs  the  parents  contribute 
1/2,  the  grandparents  1/3,  the  great  grand- 
parents 2/9,  etc. 

Theoretically     the     number     of     ancestors 


cf 

; 

d* 

? 

d1 

p 

d1 

? 

d1 

5 

d1 

? 

d 

o 

d1   9 

<J 

? 

<? 

? 

(J 

? 

rf 

? 

j 

? 

J1 

9 

c?  9 

drrh 

i 

5 

TJT 

T!T 

h^ 

i 

5 

jj 

i 

if- 

s 

:li 

TJirlr 

Parents 

Grand  Pts 

Gt  Gd  Pts 
Q\  Gd  P'ts 


FIG.  46.  Diagram  of  Gallon's  "Law  of  Ancestral  Inheri- 
tance." The  whole  heritage  is  represented  by  the  entire  rect- 
angle; that  derived  from  each  progenitor  by  the  smaller 
squares;  the  number  of  the  latter  doubles  in  each  ascending 
generation  while  its  area  is  halved.  (After  Thompson.) 

doubles  in  each  ascending  generation;  there 
are  two  parents,  four  grandparents,  eight 
great-grandparents,  etc.  If  this  continued  to 
be  true  indefinitely  the  number  of  ancestors  in 
any  ascending  generation  would  be  (2)n,  in 


PHENOMENA  OF  INHERITANCE  217 

which  n  represents  the  number  of  generations. 
There  have  been  about  57  generations  since 
the  beginning  of  the  Christian  Era,  and  if  this 
rule  held  true  indefinitely  each  of  us  would 
have  had  at  the  time  of  the  birth  of  Christ  a 
number  of  ancestors  represented  by  (2)57  or 
about  120  quadrillions, — a  number  far  greater 
than  the  entire  human  population  of  the  globe 
since  that  time.  As  a  matter  of  fact,  owing  to 
the  intermarriage  of  cousins  of  various  de- 
grees the  actual  number  of  ancestors  is  much 
smaller  than  the  theoretical  number.  For  ex- 
ample, Plate  says  that  the  present  Emperor 
of  Germany  had  only  162  ancestors  in  the 
10th  ascending  generation,  instead  of  512,  the 
theoretical  number.  Nevertheless  this  calcu- 
lation will  serve  to  show  how  widespread  our 
ancestral  lines  are,  and  how  nearly  related  are 
all  people  of  the  same  race. 

Davenport  concludes  that  no  people  of 
English  descent  are  more  distantly  related 
than  30th  cousins,  while  most  people  are  much 
more  closely  related  than  that.  If  we  allow 
three  generations  to  a  century,  and  calculate 
that  the  degree  of  cousinship  is  determined  by 


218  HEREDITY    AND    ENVIRONMENT 

the  number  of  generations  less  two,  since  first 
cousins  appear  only  in  the  third  generation, 
the  first  being  that  of  the  parents  and  the  sec- 
ond that  of  the  sons  and  daughters,  we  find 
that  30th  cousins  at  the  present  time  would 
have  had  a  common  ancestor  about  one  thou- 
sand years  ago  or  approximately  at  the  time 
of  William  the  Conqueror.  As  a  matter  of 
fact  most  persons  of  the  same  race  are  much 
more  closely  related  than  this,  and  certainly 
we  need  not  go  back  to  Adam  nor  even  to 
Shem,  Ham,  or  Japhet  to  find  our  common 
ancestor. 

2.  The  second  principle  which  Galton  de- 
duced from  his  statistical  studies  is  known  as 
the  Law  of  Filial  Regression,  or  what  might 
be  called  the  tendency  to  mediocrity.  He 
found  that,  on  the  average,  extreme  peculiari- 
ties of  parents  were  less  extreme  in  children. 
Thus,  "the  stature  of  adult  offspring  must  on 
the  whole  be  more  mediocre  than  the  stature  of 
their  parents,  that  is  to  say  more  near  to  the 
mean  or  mid  of  the  general  population";  and 
again,  "the  more  bountifully  a  parent  is 
gifted  by  nature,  the  more  rare  will  be  his 


PHENOMENA  OF  INHERITANCE  219 

good  fortune  if  he  begets  a  son  who  is  as 
richly  endowed  as  himself."  This  so-called 
law  of  filial  regression  is  represented  graphi- 
cally in  Fig.  47  in  .which  the  actual  stature 
of  individual  parents  is  shown  by  the  oblique 
line,  the  stature  of  children  by  the  dotted 
curve,  and  the  mean  stature  of  the  race  in  the 
horizontal  dotted  line. 

One  of  the  chief  aims  and  results  of  statis- 
tical study  is  to  eliminate  individual  peculiari- 
ties and  to  obtain  general  and  average  results. 
Such  work  may  be  of  great  importance  in  the 
study  of  heredity,  especially  where  questions 
of  the  occurrence  or  distribution  of  particular 
phenomena  are  concerned;  but  the  causes  of 
heredity  are  individual  and  physiological,  and 
averages  are  of  less  value  in  finding  the  causes 
of  such  phenomena  than  is  the  intensive  study 
of  individual  cases. 

By  observation  alone  it  is  usually  impossi- 
ble to  distinguish  between  inherited  and  en- 
vironmental resemblances  and  differences,  and 
yet  this  distinction  is  essential  to  any  study 
of  inheritance.  If  all  sorts  of  likenesses  and 
unlikenesses  are  lumped  together,  whether  in- 


220  HEREDITY    AND    ENVIRONMENT 


Inches 
73- 

72 

% 

71- 

/(/ 

70- 

0 

/[         y 

fiQ 

/.]•-••"" 

oy- 

Mea-r,     heiSht  of  all  jaa.rent*           „/   •' 

68- 

'ft-" 

..••x"   / 

fifi 

zz 

65 

I  / 

64- 

y_ 

fiX- 

FIG.  47.  Scheme  to  illustrate  Galton's  "Law  of  Filial  Re- 
gression" as  shown  in  the  stature  of  parents  and  children. 
The  mean  height  of  all  parents  is  shown  by  the  dotted  line 
between  68  and  69  inches.  The  circles  through  which  the 
diagonal  line  runs  represent  the  heights  of  graded  groups  of 
parents  and  the  arrow  heads  indicate  the  average  heights  of 
their  children.  The  offspring  of  undersized  parents  are  taller 
and  of  oversized  parents  are  shorter  than  their  respective 
parents.  (From  Walter.) 


PHENOMENA  OF  INHERITANCE  221 

herited  or  not,  our  study  of  inheritance  can 
only  end  in  confusion.  The  value  of  statistics 
depends  upon  a  proper  classification  of  the 
things  measured  and  enumerated,  and  if 
things  which  are  not  commensurable  are 
grouped  together  the  results  may  be  quite 
misleading  and  worthless.  Unfortunately 
Galton  and  Pearson,  as  well  as  some  of  their 
followers,  have  not  carefully  distinguished  be- 
tween hereditary  and  environmental  charac- 
ters. Furthermore  much  of  their  material 
was  drawn  from  a  general  population  in  which 
were  many  different  families  and  lines  not 
closely  related  genetically.  Consequently 
their  statistical  studies  are  of  little  value  in  dis- 
covering the  physiological  principles  or  laws 
of  heredity.  Tennings  (1910)  well  says, 
"Galton's  laws  of  regression  and  of  ancestral 
inheritance  are  the  product  mainly  of  a  lack 
of  distinction  between  two  absolutely  diverse 
things,  between  non-inheritable  fluctuations 
on  the  one  hand,  and  permanent  genotypic 
differentiations  on  the  other."  In  the  case  of 
man  we  have  few  certain  tests  to  determine 
whether  the  differential  cause  of  any  character 


222  HEREDITY  AND  ENVIRONMENT 

is  hereditary  or  environmental,  but  in  the  case 
of  animals  and  plants,  where  experiments  may 
be  performed  on  a  large  scale,  it  is  possible  to 
make  such  tests  by  (1)  experiments  in  which 
the  environment  is  kept  as  uniform  as  possible 
while  the  hereditary  factors  differ,  and  ( 2 )  ex- 
periments in  which,  in  a  series  of  cases,  the 
hereditary  factors  are  fairly  constant  while 
the  environment  differs.  In  this  way  the  dif- 
ferential cause  or  causes  of  any  character  may 
be  located  in  heredity,  in  environment  or  in 
both. 

The  observational  and  statistical  study  of 
inheritance  helped  to  outline  the  problem  but 
did  little  to  solve  it.  Certain  phenomena  of 
hereditary  resemblance  between  ascendants 
and  descendants  were  made  intelligible,  but 
there  were  many  peculiar  and  apparently  ir- 
regular or  lawless  phenomena  which  could  not 
be  predicted  before  they  occurred  nor  ex- 
plained afterward.  For  example  when  Dar- 
win crossed  different  breeds  of  domestic 
pigeons,  no  one  of  which  had  a  trace  of  blue 
in  its  plumage,  he  sometimes  obtained  off- 
spring with  more  or  less  of  the  blue  color  and 


PHENOMENA  OF  INHERITANCE  223 

markings  of  the  wild  rock  pigeon  from  which 
domestic  pigeons  are  presumably  descended. 
He  described  many  cases  of  dogs,  cattle  and 
swine,  as  well  as  many  cultivated  plants,  in 
which  offspring  resembled  distant  ancestors 
and  differed  from  nearer  ones;  such  cases  had 
long  been  known  and  were  spoken  of  as  "re- 
versions." He  observed  many  cases  in  which 
certain  characters  of  one  parent  prevailed  over 
corresponding  characters  of  the  other  parent 
in  the  offspring,  this  being  known  as  "pre- 
potency"; but  there  was  no  satisfactory  ex- 
planation of  these  curious  phenomena.  They 
did  not  come  under  either  of  Galton's  laws, 
and  their  occurrence  was  apparently  so  ir- 
regular that  every  such  case  seemed  to  be  a 
law  unto  itself. 


224  HEREDITY  AND  ENVIRONMENT 


C.  EXPERIMENTAL   STUDY  OF   INHERI- 
TANCE 

I.    MENDELISM 

The  year  1900  marks  the  beginning  of  a 
new  era  in  the  study  of  inheritance.  In  the 
spring  of  that  year  three  botanists,  deVries, 
Correns,  and  Tschermak,  discovered  inde- 
pendently an  important  principle  of  heredity 
and  at  the  same  time  brought  to  light  a  long 
neglected  and  forgotten  work  on  "Experi- 
ments in  Plant  Hybridization"  by  Gregor 
Mendel,  in  which  this  same  principle  was  set 
forth  in  detail.  This  principle  is  now  gener- 
ally known  as  "Mendel's  Law."  Mendel,  who 
was  a  monk,  and  later  abbot,  of  the  Konigs- 
kloster,  an  Augustinian  monastery  in  Briinn, 
Austria,  published  the  results  of  his  experi- 
ments on  hybridization  in  the  Proceedings  of 
the  Natural  History  Society  of  Briinn  in 
1866.  The  paper  attracted  but  little  attention 
at  the  time  although  it  contained  some  of  the 
most  important  discoveries  regarding  inheri- 
tance which  had  ever  been  made,  and  it  re- 


PHENOMENA  OF  INHERITANCE  225 

mained  buried  and  practically  unknown  for 
thirty-five  years.  Plant  hybridization  had 
been  studied  extensively  before  Mendel  began 
his  work,  but  he  carried  on  his  observations  of 
the  hybrids  and  of  their  progeny  for  a  longer 
time  and  with  greater  analytical  ability  than 
any  previous  investigator  had  done.  The 
methods  and  results  of  his  work  are  so  well 
known  through  the  writings  of  Bateson,  Pun- 
nett,  and  many  others  that  it  is  unnecessary 
to  dwell  at  length  upon  them  here.  In  brief 
Mendel's  method  consisted  in  crossing  two 
forms  having  distinct  characters,  and  then  in 
counting  the  number  of  offspring  in  successive 
generations  showing  one  or  the  other  of  these 
characters. 

During  the  eight  years  preceding  the  publi- 
cation of  his  paper  in  1866  Mendel  hybridized 
some  twenty-two  varieties  of  garden  peas. 
This  group  of  plants  was  chosen  because  the 
different  varieties  could  be  cross-fertilized  or 
self-fertilized  and  were  easily  protected  from 
the  influence  of  foreign  pollen;  because  the 
hybrids  and  their  offspring  remained  fertile 
through  successive  generations;  and  because 


226  HEREDITY  AND  ENVIRONMENT 

the  different  varieties  are  distinguished  by 
constant  differentiating  characters.  Mendel 
devoted  his  attention  to  seven  of  these  charac- 
ters, which  he  followed  through  several  gener- 
ations of  hybrids,  viz., 

(1)  Differences   in   the   form   of   the   ripe 
seeds,  whether  round  or  wrinkled. 

(2)  Differences   in  the  color  of  the   food 
material  within  the  seeds,  whether  pale  yel- 
low, orange  or  green. 

(3)  Differences   in   the   color   of  the   seed 
coats  (and  in  some  cases  of  the  flowers  also) , 
whether  white,  gray,  gray  brown,   leather 
brown,  with  or  without  violet  spots. 

(4)  Differences   in   the   form   of   the   ripe 
pods,  whether  simply  inflated  or  constricted 
between  the  seeds. 

(5)  Differences  in  the  color  of  the  unripe 
pods,  whether  light  to  dark  green,  or  vividly 
yellow. 

(6)  Differences    in    the    positions    of    the 
flowers,  whether  axial,  that  is   distributed 
along  the  stem,  or  terminal,  that  is  bunched 
at  the  top  of  the  stem. 


PHENOMENA  OF  INHERITANCE  227 

(7)   Differences  in  the  length  of  the  stem, 

whether  tall  or  short. 

1.  Results  of  Crossing  Individuals  with  one 
.  Pair  of  Contrasting  Characters. — Having  de- 
termined that  these  characters  were  constant 
for  certain  varieties  or  species  Mendel  then 
proceeded  to  cross  one  variety  with  another,  by 
carefully  removing  the  unripe  stamens,  with 
their  pollen,  from  the  flowers  of  one  variety 
and  dusting  upon  the  stigmas  of  such  flowers 
the  pollen  of  a  different  variety.  In  this  way 
he  crossed  varieties  of  peas  which  differed  from 
each  other  in  some  one  of  the  characters  men- 
tioned above,  and  then  studied  the  offspring  of 
several  successive  generations  with  respect  to 
this  character. 

In  every  case  he  discovered  that  the  plants  ' 
that  developed  from  such  a  cross  showed  only 
one  of  the  two  contrasting  characters  of  the 
parent  plants,  i.e.  all  were  round-seeded,  yel- 
low-seeded, tall,  etc.,  although  one  of  the 
parents  had  wrinkled  seeds,  green  seeds,  or 
short  stem,  etc.  "Those  characters  which  are 
transmitted  entire  or  almost  unchanged  in  the 
hybridization  are  termed  dominant,  and  those 


228  HEREDITY  AND  ENVIRONMENT 

which  become  latent  in  the  process,  recessive/' 
These  hybrids1  when  self-fertilized  gave  rise 
to  a  second  filial  generation  of  individuals 
some  of  which  showed  the  dominant  character 
and  others  the  recessive,  the  relative  numbers 
of  the  two  being  approximately  three  to  one. 
Thus  the  hybrids  produced  by  crossing  yellow- 
seeded  and  green-seeded  peas  yielded  when 
self-fertilized  6,022  yellow  seeds  and  2,001 
green  seeds,  or  very  nearly  three  yellow  to 
one  green  (Fig.  48).  The  hybrids  produced 
by  crossing  round  and  wrinkled  seeded  varie- 
ties yielded  in  the  second  filial  generation  5,474 
round  and  1,850  wrinkled  seeds,  or  approxi- 
mately three  round  to  one  wrinkled  (Fig. 
52) .  The  hybrids  from  tall-stemmed  and  short- 
stemmed  parents  produced  in  the  second  filial 
generation  787  long-stemmed  and  277  short- 
stemmed  plants,  or  again  approximately  three 


1  Bateson  introduced  the  term  "homozygote"  for  pure-bred 
individuals  resulting  from  the  union  of  gametes  which  are 
hereditarily  similar,  and  heterozygote  for  hybrids  resulting 
from  the  union  of  hereditarily  dissimilar  gametes*  The 
gametes  formed  from  a  homozygote  are  all  of  the  same 
hereditary  type,  those  formed  from  a  heterozygote  are  of  two 
or  more  different  types. 


PHENOMENA  OF  INHERITANCE 


FIG.  48.  Diagram  showing  the  results  of  crossing  yellow- 
seeded  (lighter  colored)  and  green-seeded  (darker  colored) 
peas.  (From  Morgan  after  Thompson.) 


230  HEREDITY    AND    ENVIRONMENT 

tall  to  one  short.  And  in  every  other  case 
Mendel  found  that  the  ratio  of  dominants  to 
recessives  in  the  second  filial  generation  was 
approximately  three  to  one.  These  recessives 
derived  from  hybrid  parents  are  pure  and  are 
known  as  "extracted"  recessives;  when  self- 
fertilized  they  produce  recessives  indefinitely. 
One-third  of  the  dominants  are  also  pure  hom- 
ozygotes,  or  "extracted"  dominants,  and  when 
self-fertilized  produce  pure  dominants  indefi- 
nitely. On  the  other  hand  two-thirds  of  the 
dominants  are  heterozygotes  and  when  self- 
fertilized  give  rise  in  the  next  generation  to 
pure  dominants,  mixed  dominant-recessives 
and  pure  recessives  in  the  proportion  of 
1  :  2  :  1.  These  general  results  are  sum- 
marized in  the  accompanying  diagram  (Fig. 
49 )  in  which  dominant  characters  are  indicated 
by  the  letter  D,,  recessive  characters  by  R,  and 
mixed  dominant-recessives,  with  the  recessive 
character  unexpressed,  by  D  (R)  ;  while  DD 
or  RR  indicate  extracted  dominants  or  reces- 
sives, that  is,  pure  dominants  or  recessives 
which  have  separated  out  from  mixed  domi- 
nant-recessives, D  (-K).  The  parental  gener- 


PHENOMENA  OF  INHERITANCE  231 

ation  is  indicated  by  the  letter  P,  and  the  suc- 
cessive filial  generations  by  Flt  F2,  F3f  etc. 

In  the  case  of  the  peas  studied  by  Mendel 
the  hybrids  of  the  Fi  generation  show  only  the 
dominant  character,  the  contrasted  recessive 
character  being  present  but  not  expressed. 
However  in  certain  cases  it  has  been  found 
that  the  hybrids  differ  from  either  parent  and 
in  successive  generations  split  up  into  both 
parental  types  and  into  the  hybrid  type;  thus 
Correns  found  that  when  a  white-flowered  va- 
riety of  Mirabilis.,  the  four  o'clock,  was  crossed 
with  a  red-flowered  variety  all  of  the  hybrids 

D        R  D         R 

Parent     Generation     ^^O  •  X  O      Homozygotes 

D(]g#         Heterozygotc*  . 


FIG.  49.  Diagram  showing  results  of  Mendelian  splitting 
where  the  parents  are  pure  dominants  and  pure  recessives 
(homozygotes).  All  pure  dominants  are  represented  by  black 
circles,  all  pure  recessives  by  white  ones,  while  mixed  domi- 
nant-recessives  (heterozygotes)  are  represented  by  circles  half 
white  and  half  black.  Successive  generations  are  marked  Fl5 
F2,  F,,  etc. 


232 


HEREDITY  AND  ENVIRONMENT 


in  the  FI  generation  had  pink  flowers  and  from 
these  in  the  F2  generation  there  came  white- 
flowered,  pink-flowered  and  red-flowered  forms 
in  the  proportion  of  1  white  :  2  pink  :  1  red, 
as  shown  in  Fig.  50.  This  is  a  better  illustra- 
tion of  Mendel's  principle  of  splitting  than  is 


FIG.  50.  Results  of  crossing  white-flowered  and  red-flow- 
ered races  of  Mirabilis  Jalapa  (four  o'clocks)  giving  a  pink 
hybrid  in  F,,  which  when  inbred  gives  in  F2  1  white,  2  pink, 
1  red.  (From  Morgan,  after  Correns.) 


PHENOMENA  OF   INHERITANCE  233 

offered  by  the  peas,  since  in  this  case  the  mixed 
dominant-recessives  D  (R)  are  always  distin- 
guishable from  the  pure  dominants  DD. 

In  the  F2  generation  and  in  all  subsequent 
ones  the  pure  dominants  and  the  pure  reces- 
sives  always  breed  true  when  self-fertilized, 
whereas  the  mixed  dominant-recessives  con- 
tinue to  split  up  in  each  successive  generation 
into  pure  dominants,  mixed  dominant-reces- 
sives and  pure  recessives  in  the  proportion 
1:2:1.  The  result  of  this  is  that  the  relative 
number  of  dominants  and  recessives  increases 
in  successive  generations,  whereas  the  relative 
number  of  mixed  dominant-recessives  de- 
creases, and  in  a  few  generations  a  hybrid  race 
will  revert  in  large  part  to  its  parental  types 
if  continued  hybridization  is  prevented.  On 
the  other  hand  there  is  no  tendency  for  the 
relative  number  of  dominants  to  increase  and 
of  recessives  to  decrease  in  successive  genera- 
tions ;  an  equal  number  of  pure  dominants  and 
pure  recessives  is  produced  in  each  generation. 

With  remarkable  insight  Mendel  recog- 
nized that  the  real  explanation  of  the  splitting 
of  pure  recessives  and  pure  dominants  from 


234  HEREDITY  AND  ENVIRONMENT 

hybrid  parents  must  be  found  in  the  composi- 
tion of  the  male  and  female  sex  cells.  Since 
such  extracted  dominants  and  recessives  breed 
true,  just  as  pure  species  do,  it  must  be  that 
their  germ  cells  are  pure.  In  the  cross  be- 
tween pure  races  of  white-flowered  and  red- 
flowered  Mirabilis  the  germ  cells  which  unite  in 
fertilization  must  be  pure  with  respect  to  white 
and  red,  though  the  individual  which  develops 
from  this  cross  is  a  pink  hybrid.  But  the  fact 
that  one-quarter  of  the  progeny  of  this  hybrid 
are  pure  white,  and  another  quarter  pure  red, 
and  that  these  thereafter  breed  true,  proves 
that  the  hybrid  produces  germ  cells  which  are 
pure  with  respect  to  red  and  white.  Further- 
more the  fact  that  one-half  the  progeny  of  this 
hybrid  are  themselves  hybrid  may  be  explained 
by  assuming  that  they  were  produced  by  the 
union  of  germ  cells  carrying  pure  white  and 
pure  red,  as  in  the  parental  generation. 

Mendel  therefore  concluded  that  individual 
germ  cells  are  always  pure  with  respect  to  any 
pair  of  contrasting  characters,  even  though 
those  germ  cells  have  come  from  hybrids  in 
which  the  contrasting  characters  are  mixed. 


PHENOMENA  OF  INHERITANCE  235 

A  single  germ  cell  can  carry  the  factors,  or 
causes,  for  red  flowers  or  white  flowers,  for 
green  seeds  or  yellow  seeds,  for  tall  stem  or 
short  stem,  etc.,  but  not  for  both  pairs  of  these 
contrasting  characters.  The  hybrids  formed 
by  crossing  white  and  red  four  o'clocks  carry 
the  factors  for  both  white  and  red,  but  the  in- 
dividual germ  cells  formed  by  such  a  hybrid 
carry  the  factors  for  white  or  red,  but  not  for 
both ;  these  factors  segregate  or  separate  in  the 
formation  of  the  germ  cells  so  that  one-half  of 
all  the  germ  cells  formed  carry  the  factor  for 
white  and  the  other  half  that  for  red. 

This  is  the  most  important  part  of  Mendel's 
Law, — the  central  doctrine  from  which  all 
other  conclusions  of  his  radiate.  It  explains 
not  only  the  segregation  of  dominant  and  re- 
cessive characters  from  a  hybrid  in  which  both 
are  present,  but  also  the  relative  numbers  of 
pure  dominants,  pure  recessives  and  mixed 
dominant-recessives  in  each  generation.  For 
if  all  germ  cells  are  pure  with  respect  to  any 
particular  character  the  hybrid  offspring  of 
any  two  parents  with  contrasting  characters 
will  produce  in  equal  numbers  two  classes  of 


236 


HEREDITY  AND  ENVIRONMENT 


germ  cells,  one  bearing  the  dominant  and  the 
other  the  recessive  factor,  and  the  chance  com- 
bination of  these  two  classes  of  male  and  fe- 
male gametes  will  yield  on  the  average  one 
union  of  dominant  with  dominant,  two  unions 
of  dominant  with  recessive  and  one  union  of 
recessive  with  recessive,  thus  producing  the 
typical  Mendelian  ratio,  IDD  :  2D(R)  : 
I  RR,  as  shown  in  the  accompanying  diagram, 
(Fig.  51  a,  b). 


€>©€>© 


•••3 


& 


00  (00 


FIG.  51.  Diagram  of  Mendelian  inheritance,  in  which  the 
individual  is  represented  by  the  large  circle,  the  germ  cells 
by  the  small  ones,  dominants  being  shaded  and  recessives 
white,  a,  Pure  dominant  X  pure  recessive  =  all  dominant- 
recessives,  b,  Dominant-recessive  X  dominant-recessive  =  1 
pure  dominant  :  2  dominant-recessives  :  1  pure  recessive, 
c,  Dominant-recessive  X  pure  dominant  =  2  pure  dominant  : 
2  dominant  =  recessive,  d,  Dominant-recessive  X  pure  reces- 
sive —  2  dominant-recessive  :  2  pure  recessive. 


PHENOMENA  OF  INHERITANCE  237 

Other  Mendelian  Ratios 

When  a  pure  dominant  is  crossed  with  a 
mixed  dominant-recessive  all  the  offspring 
show  the  dominant  character,  though  one-half 
are  pure  dominants  and  the  other  half  domi- 
nant-recessives.  Thus  if  a  pure  round-seeded 
variety  of  pea  is  crossed  with  a  hybrid  between 
a  round-  and  a  wrinkled-seeded  one,  all  the 
progeny  are  round-seeded,  though  one  half  of 
them  carry  the  factor  for  wrinkled  seed;  this 
may  be  graphically  represented  as  follows, 
R  representing  the  factor  for  round  seed 
and  W  that  for  wrinkled  seed: 

$    germ  cells  R  \    ,  R 

?    germ  cells  R  /\  W 
Possible  combinations  2  RR:  2  R  (W). 

In  subsequent  generations  the  progeny  of  the 
pure  round  (RR)  breed  true  and  produce 
only  round-seeded  peas,  whereas  the  progeny 
of  the  hybrid  round-wrinkled  (RW)  split 
up  into  pure  round,  hybrid  round-wrinkled, 
and  pure  wrinkled  in  the  regular  Mendelian 
ratio  oflRR:2R(W)  :  1  WW  (Fig.  52). 


238  HEREDITY  AND  ENVIRONMENT 

When  a  pure  recessive  is  crossed  with  a 
mixed  dominant-recessive  another  typical  ratio 
results.  Thus  if  a  wrinkled-seeded  variety  of 
pea  is  crossed  with  a  hybrid  between  a  round- 
and  wrinkled-seeded  one,  round-seeded  and 
wrinkled-seeded  peas  are  produced  in  the  pro- 
portion of  1  :  1.  This  is  due  to  the  fact  that 
the  hybrid  produces  two  kinds  of  germ  cells, 
the  pure-bred  but  one,  and  the  possible  combi- 
nations of  these  are  as  follows: 

?    germ  cells  W  \  /    W 


$    germ  cells  R         ^  W 
Possible  combinations  2  R  W:  2  WW . 

This  ratio  of  1  :  1  is  approximately  the  ratio 
of  the  two  sexes  in  many  animals  and  plants, 
and  there  is  good  reason  to  believe  that  sex  is 
a  Mendelian  character  of  this  sort,  in  which 
one  parent  is  heterozygous  for  sex  and  the 
other  homozygous  (Fig.  51,  d). 

2.  Results  of  Crossings  where  there  is  more 
than  one  Pair  of  Contrasting  Characters. — It 
rarely  happens  that  two  individuals  differ  in  a 
single  character  only;  more  frequently  they 
differ  in  many  characters,  and  this  leads  to  a 


PHENOMENA   OF   INHERITANCE  239 

great  increase  in  the  number  of  types  of  off- 
spring in  the  F2  generation.  t  But  however 
many  pairs  of  contrasting  characters  the  par- 
ents may  show  each  pair  may  be  considered  by 
itself  as  if  it  were  the  only  contrasting  pair, 
and  when  this  is  done  all  the  offspring  may  be 
classified  according  to  the  regular  Mendelian 
formula  given  above. 

But  when  two  or  more  contrasting  charac- 
ters of  the  parents  are  followed  to  the  F2  gen- 
eration many  permutations  of  these  characters 
occur,  thus  giving  rise  to  a  larger  number  of 
types  of  individuals  than  when  a  single  pair  of 
characters  is  concerned.  When  there  is  only 
one  pair  of  contrasting  characters  there  are 
usually  but  two  types  of  offspring  apparent  in 
the  F2  generation,  viz.,  dominants  and  reces- 
sives  in  the  ratio  of  3:1  (Fig.  52) ;  where  there 
are  two  pairs  of  contrasting  characters  in  the 
parents  there  are  four  types  of  offspring  in 
the  Fo  generation  in  the  ratio  of  (3  :  I)2  = 
9:3:3:1;  when  there  are  three  pairs  of  con- 
trasting characters  in  the  parents  there  are 
eight  types  of  offspring  apparent  in  the  F2 
generation  in  the  proportions  of  (3  :  I)3  = 


240  HEREDITY  AND  ENVIRONMENT 

27:9:9:9:3:3:3:1,  etc.  Thus  when  Mendel 
crossed  a  variety  of  peas  bearing  round  and 
yellow  seeds  with  another  variety  having 
wrinkled  and  green  seeds  all  the  offspring  of 
the  Fi  generation  bore  round  and  yellow  seeds, 
round  being  dominant  to  wrinkled,  and  yellow 
to  green.  But  the  plants  raised  from  these 
seeds,  when  self-fertilized,  yielded  seeds  of 


© 


FIG.  52.  Monohybrid  Diagram  showing  results  of  crossing 
round  (R)  seeded  with  wrinkled  (W)  seeded  peas.  Large  cir- 
cles represent  zygotes,  small  ones,  or  single  letters,  gametes. 
In  Fj  all  individuals  are  round  but  contain  round  and  wrinkled 
gametes.  In  F2  the  g  gametes  are  placed  above  the  square, 
the  $  ones  to  the  left,  and  the  possible  combinations  of  £ 
and  5  gametes  are  shown  in  the  small  squares,  the  relative 
numbers  of  different  types  being  1  RR:2  R(W):1  WW. 


PHENOMENA  OF  INHERITANCE 


241 


four  types,  round  and  yellow  (RY),  wrinkled 
and  yellow  (WY),  round  and  green  (RG), 
and  wrinkled  and  green  (WG)  in  the  propor- 
tion of  9  :  3  :  3  :  1  as  shown  in  Fig.  53. 

In  this  case  also  this  ratio  may  be  explained 


YRCGW) 


2T7 

YR 


YR       YW       GR        GW 


YW 


GR 


GW 


F^ 


FIG.  53.  Dihybrid  Diagram  showing  results  of  crossing  peas 
having  yellow-round  (YR)  seeds  with  others  having  green- 
wrinkled  (GR)  ones.  Four  types  of  germ  cells  are  formed 
by  such  a  hybrid,  viz.  YR,  YW,  GR,  GW,  and  the  16  possible 
combinations  (genotypes)  of  these  $  and  $  gametes  are  shown 
in  the  small  squares.  Since  recessive  characters  do  not  ap- 
pear when  mated  with  dominant  ones  these  16  genotypes  pro- 
duce 4  phenotypes  in  the  following  relative  numbers:  9YR: 
3YW:3GR:1GW.  There  is  1  pure  dominant  (upper  left  cor- 
ner), 1  pure  recessive  (lower  right  corner),  4  homozygotes  in 
diagonal  line  between  these  corners,  and  12  heterozygotes. 


242  HEREDITY  AND  ENVIRONMENT 

by  assuming  that  the  germ  cells  (ovules  and 
pollen)  are  pure  with  respect  to  each  of  the 
contrasting  characters,  round-wrinkled,  yel- 
low-green, and  therefore  any  combination  of 
these  may  occur  in  a  germ  cell  except  the  com- 
binations EW  and  YG.  Accordingly  there 
are  four  possible  kinds  of  germ  cells  as  follows : 

Y  G 

V         i.e.  YH,  YW,  GR,  GW. 

R  /    \  W 

Each  of  these  four  kinds  of  pollen  may  fertilize 
any  of  the  four  kinds  of  ovules,  thus  giving 
rise  to  sixteen  combinations,  no  two  of  which 
are  alike,  as  shown  in  Fig.  53.  The  dominant 
characters  are  in  this  case  round  and  yellow, 
and  only  when  one  of  these  is  absent  can  its 
contrasting  character,  wrinkled  or  green, 
develop.  Accordingly  the  sixteen  possible 
combinations  yield  seeds  of  four  different  ap- 
pearances and  in  the  following  proportions: 
9RY:3RG:3WY:IWG.  Only  one  individual 
in  each  of  these  four  classes  is  pure  (homozy- 
gous)  and  continues  to  breed  true  in  successive 
generations;  in  Fig.  53  these  are  found  in 
the  diagonal  from  the  upper  left  to  the  lower 


PHENOMENA  OF  INHERITANCE  243 

right  corner.  All  other  individuals  are  heter- 
ozygous and  show  Mendelian  splitting  in  the 
next  generation. 

When  parents  differ  in  three  contrasting 
characters  a  much  larger  number  of  combina- 
tions is  possible  in  the  F2  generation.  Thus 
if  a  pea  with  round  (R)  and  yellow  (Y) 
seeds,  and  with  tall  (T)  stem  is  crossed  with 
one  having  wrinkled  (W)  and  green  (G) 
seeds,  and  dwarf  ( D)  stem  all  the  progeny  of 
the  FI  generation  have  round  and  yellow  seeds 
and  tall  stem,  It,  Y,  and  T  being  dominant 
to  W,  G,  and  D.  But  in  the  F2  generation 
there  are  sixty-four  possible  combinations 
(genotypes)  of  these  six  characters;  but  since 
a  recessive  character  does  not  develop  if  its 
contrasting  dominant  character  is  present  there 
are  only  eight  types  which  come  to  expression 
(phenotypes)  and  in  the  following  numbers: 
27RYT  :  9RYD  :  9RGT  :  3RGD  :  9WYT 
:  3WYD  :  3WGT  :  1WGD.  Of  these  sixty- 
four  genotypes  only  eight  are  homozygous  and 
breed  true  (those  lying  in  the  diagonal  be- 
tween upper  left  and  lower  right  corners  in 
Fig.  54),  while  only  one  is  pure  dominant 


244  HEREDITY  AND  ENVIRONMENT 

and  one  pure  recessive  (the  ones  in  the  upper 
left  and  lower  right  corners  of  Fig.  54) . 

When  the  parents  differ  in  one  character 
only,  the  offspring  formed  by  their  crossing 
are  called  monohybrids,  when  there  are  two 
contrasting  characters  in  the  parents  the  off- 
spring are  dihybrids,  when  three,  trihybrids, 
and  when  the  parents  differ  in  more  than  three 
characters  the  offspring  are  called  polyhybrids. 
There  are  certainly  few  cases  in  which  parents 
actually  differ  in  only  a  single  character,  but 
since  each  contrasting  character  may  be  dealt 
with  separately,  as  if  it  were  the  only  one,  and 
since  the  number  of  types  of  offspring  in- 
creases greatly  when  more  than  one  or  two 
characters  are  considered  at  the  same  time,  it 
is  customary  to  deal  simultaneously  with  only 
one  or  two  characters  of  hybrids,  even  though 
the  parents  may  have  differed  in  many 
characters. 

3.  Inheritance  Formulae. — Mendel  repre- 
sented the  hereditary  constitution  of  the  plants 
used  in  his  experiments  by  letters  employed 
as  symbols,  dominant  characters  being  repre- 
sented by  capitals  and  recessives  by  small  let- 


RYT 


RYT       RYD        RGT       ROD       WYT      WYO       WGT       WGD 


FIG.  54.  Trihybrid  Diagram  showing  results  of  crossing 
peas  having  round-yellow  seeds  and  tall  stem  (RYT)  with 
peas  having  wrinkled-green  seeds  and  dwarf  stem  (WGD). 
Eight  types  of  germ  cells  result  from  such  a  hybrid,  as  shown 
in  the  <$  gametes  above  the  square  and  the  $  ones  to  the 
left  of  it,  and  the  possible  combinations  (genotypes)  of 
these  £  and  $  gametes  are  shown  in  the  64  small  squares  of 
which  only  1  is  pure  dominant  (upper  left  corner),  1  pure 
recessive  (lower  right  corner)  and  8  homozygotes  (in  di- 
agonal line  between  these  corners).  The  relative  numbers 
of  the  different  phenotypes  are  27RYT:  9RYD:  9RGT:  9 WYT: 
3RGD:  3WYD:  3WGT:  1WGD. 


246  HEREDITY  AND  ENVIRONMENT 

ters.  The  seven  contrasting  characters  of  his 
peas  could  be  represented  as  follows: 

Seeds,  round  (A),  or  wrinkled  (a) ;  yellow 
(B),  or  green  (b) ;  with  gray  seed 
coats  (C),  or  white  seed  coats  (c). 

Pods,  green  (X)),  or  yellow  (d)  ;  inflated 
(E),  or  constricted  (e). 

Habit,  tall  (F),  or  dwarf  (/). 

Flowers,  axial  (G),  or  terminal  (g). 

It  is  possible  for  one  plant  to  have  all  of 
these  dominant  characters  or  all  of  the  reces- 
sive ones,  or  part  of  one  kind  and  part  of  the 
other.  The  inheritance  formula  of  a  plant 
having  all  seven  of  the  dominant  characters  is 
ABCDEFG;  of  one  having  all  of  the  recessive 
characters  abcdefg.  When  two  such  plants 
are  crossed  the  inheritance  formula  of  the  hy- 
brid is  AaBbCcDdEeFfGg,  and  since  the 
dominant  and  recessive  characters  (or  rather 
determiners  of  characters)  represented  by 
these  seven  pairs  of  letters  separate  in  the  for- 
mation of  the  gametes,  and  since  each  separate 
determiner  may  be  associated  with  either  mem- 
ber of  the  six  other  pairs,  the  number  of  possi- 
ble combinations  of  these  determiners  in  the 


PHENOMENA  OF  INHERITANCE  247 

gametes  is  (2)7  or  128.  That  is,  in  this  case 
128  kinds  of  germ  cells  may  be  produced,  each 
having  a  different  inheritance  formula;  and 
since  each  of  these  128  kinds  of  male  germ 
cells  may  unite  with  any  one  of  the  128  kinds 
of  female  germ  cells,  the  number  of  possible 
combinations  is  (128)2  or  16,384,  which  repre- 
sents the  number  of  combinations  of  these  char- 
acters which  are  possible  in  the  F2  generation. 
Every  one  of  these  more  than  sixteen  thousand 
genotypes  may  be  represented  by  various  com- 
binations of  the  letters  ABCDEFG  and 
abcdefg. 

When  many  characters  are  concerned  it  is 
difficult  to  remember  what  each  letter  stands 
for,  and  consequently  it  is  customary  in  such 
cases  to  designate  characters  by  the  initial  let- 
ter in  the  name  of  that  character.  By  this 
form  of  short  hand  one  can  show  in  a  graphic 
way  the  possible  segregations  and  combina- 
tions of  hereditary  units  in  gametes  and  zy- 
gotes  through  successive  generations,  and  as  a 
result  many  modern  works  on  Mendelian 
inheritance  look  like  pages  of  algebraic 
formulae. 


248  HEREDITY  AND  ENVIRONMENT 

Some  progress  has  been  made,  as  was 
pointed  out  in  the  last  lecture,  in  identifying 
certain  structures  of  the  germ  cells  with  cer- 
tain hereditary  units,  but  quite  irrespective  of 
what  these  units  may  be  and  where  they  may 
be  located  it  is  possible,  by  means  of  the  Men- 
delian  theory  of  segregation  of  units  in  the 
germ  cells  and  of  chance  combinations  of  these 
in  fertilization  to  predict  the  number  of  geno- 
types and  phenotypes  which  may  be  expected 
as  the  result  of  a  given  cross. 

4.  Presence  and  Absence  Hypothesis. — 
Mendel  spoke  of  the  presence  of  contrasting 
or  differentiating  characters  in  the  plants 
which  he  crossed,  such  as  round  or  wrinkled 
seeds,  tall  or  short  stems,  etc.  Many  other 
writers  regard  these  contrasting  characters  as 
positive  and  negative  expression  of  a  single 
character,  and  consequently  they  speak  of  the 
presence  or  absence  of  single  characters:  thus 
round  seeds  are  due  to  the  presence  of  a  factor 
for  roundness  (A)  while  wrinkled  seeds  are 
characterized  by  the  absence  of  that  factor 
(a).  Round  seeds  are  wrinkled  seeds  plus  the 
factor  for  roundness.  Most  of  the  phenomena 


PHENOMENA  OF  INHERITANCE  249 

of  Mendelian  inheritance  are  more  simply 
stated  in  terms  of  presence  or  absence  of 
single  characters  than  in  terms  of  contrasting 
characters. 

When  both  gametes  carry  similar  positive 
factors  the  zygote  has  a  "double  dose"  of  such 
factors  and  is  said  to  be  duplex;  when  only  one 
of  the  gametes  carries  such  a  factor  the  zygote 
has  a  "single  dose"  and  is  simplex,  when 
neither  gamete  carries  a  positive  factor  or  fac- 
tors, the  zygote  receives  only  negative  factors 
and  is  said  to  be  nulliplex.  Thus  the  union  of 
gametes  AB  (?)  and  AB  (<?)  yields  zygote 
A  ABB,  which  is  duplex  in  constitution; 
gametes  Ab  (?)  and  aB  (<?)  yield  zygote 
AaBb,  which  is  simplex;  gametes  ab  (?)  and 
ab  (<?)  yield  zygote  aabb,  which  is  nulliplex. 

In  some  instances  a  character  comes  to  full 
expression  only  when  it  is  derived  from  both 
parents,  that  is,  when  it  is  duplex;  if  derived 
from  one  parent  only,  that  is,  if  simplex,  it  is 
diluted  in  appearance  and  is  intermediate  be- 
tween the  two  parents.  For  example,  when 
white-flowered  four  o'clocks  which  are  nulli- 
plex are  crossed  with  red-flowered  ones  which 


250  HEREDITY  AND  ENVIRONMENT 

are  duplex  the  progeny,  which  are  simplex, 
bear  pink  flowers;  in  this  case  red  flowers  are 
produced  only  when  the  factor  for  red  is  de- 
rived from  both  parents,  pink  flowers  when  it 
is  derived  from  one  parent,  white  flowers  when 
it  is  derived  from  neither  parent  ( Fig.  50 ) . 

5.  Summary  of  Mendelian  Principles. — 
Since  the  rediscovery  in  1900  of  Mendel's  work 
many  investigators  have  carried  out  similar  ex- 
periments on  many  species  of  animals  and 
plants  and  have  greatly  extended  our  knowl- 
edge of  the  principles  of  inheritance  discovered 
by  Mendel,  but  in  the  main  Mendel's  conclu- 
sions have  been  confirmed  again  and  again,  so 
that  there  is  no  doubt  that  they  constitute  an 
important  rule  of  inheritance  among  all 
organisms. 

In  brief  the  "Mendelian  Law  of  Alternative 
Inheritance"  or  of  hereditary  "splitting"  con- 
sists of  the  following  principles: 

(a)  The  principle  of  unit  characters. — The 
total  heritage  of  an  organism  may  bq  analyzed 
into  a  number  of  characters  which  are  inherited 
as  a  whole  and  are  not  further  divisible ;  these 
are  the  so-called  "unit  characters"  (deVries). 


PHENOMENA  OF  INHERITANCE  251 

(b)  The  principle   of  dominance. — When 
contrasting  unit  characters  are  present  in  the 
parents  they  do  not  as  a  rule  blend  in  the 
offspring,  but  one  is  dominant  and  usually  ap- 
pears fully  developed,  while  the  other  is  reces- 
sive and  temporarily  drops  out  of  sight. 

(c)  The  principle  of  segregation. — Every 
individual  germ  cell  is  "pure"  with  respect  to 
any  given  unit  character,  even  though  it  come 
from  an  "impure"  or  hybrid  parent.     In  the 
germ  cells  of  hybrids  there  is  a  separation  of 
the  determiners  of  contrasting  characters  so 
that  different  kinds   of  germ  cells   are  pro- 
duced, each  of  which  is  pure  with  regard  to 
any  given  unit  character.    This  is  the  principle 
of  segregation  of  unit  characters,  or  of  the 
"purity"  of  the  germ  cells.     Every  sexually 
produced  individual  is  a  double  being,  double 
in   every   cell,   one-half  having  been   derived 
from  the  male  and  the  other  half  from  the  fe- 
male sex  cell.     This  double  being,  or  zygote, 
again  becomes  single  in  the  formation  of  the 
germ  cells  only  once  more  to  become  double 
when  the  germ  cells  unite  in  fertilization. 


252  HEREDITY  AND  ENVIRONMENT 


II.  MODIFICATIONS  AND  EXTENSIONS  OF  MEN- 
DELIAN  PRINCIPLES 

It  is  a  common  experience  that  natural 
phenomena  are  found  to  be  more  complex  the 
more  thoroughly  they  are  investigated.  Na- 
ture is  always  greater  than  our  theories,  and 
with  few  exceptions  hypotheses  which  were 
satisfactory  at  one  stage  of  knowledge  have  to 
be  extended,  modified  or  abandoned  as  knowl- 
edge increases.  This  observation  is  well  illus- 
trated in  the  case  of  the  Mendelian  theory. 
The  principles  proposed  by  Mendel  were  rela- 
tively simple,  but  in  attempting  to  apply  them 
to  the  many  phenomena  of  inheritance  now 
known  it  has  become  necessary  to  modify  or 
extend  them  in  many  ways.  And  yet  the  gen- 
eral and  fundamental  truth  of  these  principles 
has  been  established  in  a  surprisingly  large 
number  of  cases,  and  they  have  been  extended 
to  forms  of  inheritance  where  at  first  it  was 
supposed  that  they  could  not  apply. 

1.  The  Principle  of  Unit  Characters  and  of 
Inheritance  Factors. — There  has  been  much 


PHENOMENA  OF  INHERITANCE  253 

criticism  on  the  part  of  some  biologists  of  the 
principle  of  unit  characters.  It  is  said  that 
unit  characters  cannot  be  independent  and  dis- 
crete things ;  the  organism  itself  is  a  unity  and 
every  one  of  its  parts,  every  one  of  its  charac- 
ters, must  influence  more  or  less  every  other 
part  and  every  other  character.  Certainly  unit 
characters  cannot  be  absolutely  independent  of 
one  another;  the  various  parts  and  organs  of 
the  body,  and  even  the  organism  as  a  whole, 
are  not  absolutely  independent,  and  yet  there 
are  varying  degrees  of  independence  in  organ- 
isms, organs,  cells,  parts  of  cells,  hereditary 
units  and  characters  which  make  it  possible  for 
purposes  of  analysis  to  deal  with  these  things 
as  if  they  were  really  independent  though  we 
know  they  are  not. 

Of  course  characters  of  adult  individuals  do 
not  exist  as  such  in  germ  cells,  but  there  is  no 
escape  from  the  conclusion  that  in  the  case  of 
inherited  differences  between  mature  organ- 
isms there  must  have  been  differences  in  the 
constitution  of  the  germ  cells  from  which  they 
developed.  For  every  inherited  character 
there  must  have  been  a  germinal  cause  in  the 


254  HEREDITY  AND  ENVIRONMENT 

fertilized  egg.  This  germinal  cause,  whatever 
it  may  be,  is  often  spoken  of  as  a  determiner 
of  a  character.  But  the  character  in  question 
is  not  to  be  thought  of  as  the  result  of  a  single 
cause  nor  as  the  product  of  the  development 
of  a  single  determiner;  undoubtedly  many 
causes  are  involved  in  the  development  of 
every  character,  but  the  differential  cause  or 
combination  of  causes  is  that  which  is  peculiar 
to  the  development  of  each  particular 
character. 

Again  it  is  not  necessary  to  suppose  that 
every  developed  character  is  represented  in  the 
germ  by  a  distinct  determiner,  or  inheritance 
unit,  just  as  it  is  not  necessary  to  suppose  that 
every  chemical  compound  contains  a  peculiar 
chemical  element ;  but  it  is  necessary  to  suppose 
that  each  hereditary  character  is  caused  by 
some  particular  combination  of  inheritance 
units  and  that  each  compound  is  produced  by 
some  particular  combination  of  chemical  ele- 
ments. An  enormous  number  of  chemical 
compounds  exists  as  the  result  of  various  com- 
binations of  some  eighty  different  elements, 
and  an  almost  endless  number  of  words  and 


PHENOMENA  OF  INHERITANCE  255 

combinations  of  words — indeed  whole  litera- 
tures— may  be  made  with  the  twenty- six  let- 
ters of  the  alphabet.  It  is  quite  probable  that 
the  kinds  of  inheritance  units  are  few  in  num- 
ber as  compared  with  the  multitudes  of  adult 
characters,  and  that  different  combinations  of 
the  units  give  rise  to  different  adult  characters ; 
but  it  is  certain  that  every  inherited  difference 
in  adult  organization  must  have  had  some 
differential  cause  or  factor  in  germinal 
organization. 

Mendel  did  not  speculate  about  the  nature 
of  hereditary  units  though  he  evidently  con- 
ceived that  there  was  something  in  the  germ 
which  corresponded  to  each  character  of  the 
plant.  Weismann  postulated  a  determinant  in 
the  germ  for  every  character  which  is  inde- 
pendently heritable,  and  many  recent  students 
of  heredity  hold  a  similar  view. 

But  it  is  evident  that  there  is  not  an  exact 
one  to  one  correspondence  of  inheritance  units 
and  adult  characters.  Many  characters  may 
be  determined  by  a  single  unit  or  factor ;  for  ex- 
ample, all  the  numerous  secondary  sexual  char- 
acters which  distinguish  males  from  females 


256  HEREDITY  AND  ENVIRONMENT 

may  be  determined  by  the  original  factor  which 
determines  whether  the  germ  cells  shall  be  ova 
or  spermatozoa. 

On  the  other  hand  two  or  more  factors  may 
be  concerned  in  the  production  of  a  single 
character.  In  many  cases  among  both  plants 
and  animals  the  development  of  color  appears 
to  depend  upon  the  presence  in  the  germ  cells 
and  the  cooperation  in  development  of  at  least 
two  factors,  viz.  (1)  a  pigment  factor  for 
black  B,  for  brown  Br,  for  yellow  Y ' ,  for  red 
R,  etc.,  and  (2)  a  color  developer  C.  When 
both  of  these  factors  are  present  color  develops, 
when  either  one  is  absent  no  color  appears. 

Such  cases  have  been  described  for  mice, 
guinea-pigs,  and  rabbits  as  well  as  for  several 
species  of  plants.  Bateson  and  Punnett  found 
two  varieties  of  white  sweet  peas  which  were 
apparently  alike  in  every  respect  except  the 
shapes  of  their  pollen  grains,  one  of  them  hav- 
ing long  and  the  other  round  pollen.  But 
when  these  were  crossed  a  remarkable  thing 
occurred  for  the  progeny  "instead  of  being 
white  were  purple  like  the  wild  Sicilian  plant 
from  which  our  cultivated  sweet  peas  are  de- 


PHENOMENA  OF  INHERITANCE  257 

scended."  This  is  apparently  a  typical  case 
of  reversion  and  its  cause  was  found  in  the 
fact  that  at  least  two  factors  are  necessary  in 
this  case  for  the  production  of  color,  a  pig- 
ment factor  R  and  a  color  developer  C.  One 
of  these,  was  lacking  in  each  of  the  white 
parents,  their  gametic  formulae  being  Cr  and 
cR,  but  when  these  two  factors  came  together 
in  the  offspring  a  purple-flowered  type  was 
produced  with  the  gametic  formula  Cc  Rr. 
These  F2  plants  produced  colored  and  white 
F3  plants  in  the  proportion  of  9  colored  to  7 
white  and  the  colored  forms  were  of  six  dif- 
ferent kinds  (Fig.  55).  For  the  production 
of  these  six  colored  forms  five  different  factors 
must  be  present  in  the  gametes,  according  to 
Punnett,  viz. :  (1)  a  color  base  R,  (2)  a  color 
developer  C,  (3)  a  purple  factor  P,  (4)  a  light 
wing  factor  L,  (5)  a  factor  for  intense  color 
I.  When  all  of  these  factors  are  present  the 
result  is  the  purple  wild  form  with  blue 
wings,  while  the  omission  of  one  or  more  of 
these  factors  leads  to  the  production  of  six 
forms  of  colored-  and  various  types  of  white- 
flowered  plants  of  the  F2  generation. 


Egg      BLUE 

HI      DEEP  PURPLE 

FIG.  55.  Results  of  crossing  two  different  races  (A  and  B) 
of  white  sweet  peas;  all  the  F,  hybrids  (C)  are  purple  with 
blue  wings  like  the  wild  ancestral  stock;  in  F2  six  colored 
varieties  are  formed  ranging  from  purple  with  blue  wings 
(D)  to  tinged  white  (I)  and  several  kinds  (genotypes)  of 
white  varieties  (K).  (After  Punnett.) 


PHENOMENA  OF  INHERITANCE  259 

Castle   found   that   eight   different   factors 
may  be  involved  in  producing  the  coat  colors 
of  rabbits;  these  are: 
C  a  common  color  factor  necessary  to  produce 

any  color. 

B  a  factor  acting  on  C  to  produce  black. 
Br  a  factor  acting  on  C  to  produce  brown. 
Y  a  factor  acting  on  C  to  produce  yellow. 
I    a  factor  which  determines  intensity  of  color. 
U  a  factor  which  determines  uniformity  of 

color. 
A  a  factor  for  agouti,  or  wild  gray  pattern,  in 

which  the  tip  of  every  hair  is  black,  below 

which  is  a  band  of  yellow,  while  the  basal 

part  of  the  hair  is  gray. 
E  a  factor  for  the  extension  of  black  or  brown 

but  not  of  yellow. 

Plate  found  that  all  of  these  factors  except 
the  last,  E,,  are  also  involved  in  the  production 
of  the  coat  colors  of  mice.  Baur  has  recog- 
nized more  than  twenty  different  factors  for 
the  color  and  form  of  flowers  in  the  snap- 
dragon, Antirrhinum. 

These  factors  are  probably  complex  chemi- 


260  HEREDITY  AND  ENVIRONMENT 

cal  substances  which  preserve  their  individu- 
ality in  various  combinations,  just  as  groups 
of  atoms  or  radicals  do  in  chemical  reactions; 
they  may  be  dropped  out  or  added,  substituted 
or  transposed,  just  as  chemical  radicals  may 
be  in  chemical  compounds.  To  this  extent 
they  maintain  continuity  and  independence, 
but  they  are  not  absolutely  independent  for 
they  react  upon  one  another  as  well  as  to  en- 
vironmental changes,  so  that  the  characters  of 
the  developed  organism  are  the  results  of  all 
these  reactions  and  interactions. 

Inheritance  Factors  and  Germinal  Units 

If  it  is  asked  whether  there  are  particular 
structures  in  germ  cells  which  correspond  to 
particular  inheritance  factors  it  must  be  ad- 
mitted that  we  have  no  certain  knowledge  on 
this  subject  and  that  opinions  differ  greatly 
with  respect  to  it.  On  the  one  hand  it  is 
maintained  that  the  entire  germ  is  concerned 
in  the  development  of  every  character,  and  on 
the  other  hand  that  the  differential  cause  of 
any  character  may  be  located  in  some  differ- 
entiated structure  or  function  of  the  germ. 


PHENOMENA  OF  INHERITANCE  261 

These  views  are  not  mutually  exclusive  and  it 
may  well  be  that  both  are  true.  We  know  that 
germ  cells  are  composed  of  many  parts  which 
differ  from  one  another  in  both  structure  and 
function,  and  it  is  highly  probable  that  there 
are  enough  of  these  parts  to  provide  a  locus 
for  every  inheritance  factor. 

There  was  a  time  when  the  cell  was  the 
Ultima  Thule  of  biological  analysis  and  when 
the  contents  of  cells  were  supposed  to  be  "per- 
fectly homogeneous,  diaphanous,  structureless 
slime."  Then  the  nucleus  was  discovered 
within  the  cell,  then  the  chromosomes  within 
the  nucleus,  then  the  chromomeres  within  the 
chromosomes,  and  there  is  no  reason  to  sup- 
pose that  organization  ceases  with  the  powers 
of  our  present  microscopes.  With  every  im- 
provement of  the  microscope  and  of  micro- 
scopical technique,  structures  have  been  found 
in  cells  which  were  undreamed  of  before,  and 
it  is  not  probable  that  the  end  has  been  reached 
in  this  regard.  We  know  that  cells  contain 
nuclei  and  chromosomes  and  chromomeres, 
centrosomes  and  plastosomes  and  microsomes, 
and  we  know  that  some  of  these  parts  differ  in 


262 


HEREDITY  AND  ENVIRONMENT 


function  as  well  as  in  structure.  And  there  is 
no  reason  to  doubt  that  if  we  had  sufficiently 
powerful  microscopes  we  should  find  still 
smaller  and  smaller  units  until  we  came  at  last 
to  molecules  and  atoms. 

The  fact  that  inheritance  units  from  the 
two  parents  unite  in  fertilization  and  later 
segregate  in  the  formation  of  gametes,  so  that 
the  latter  are  pure  with  respect  to  any  char- 
acter, is  a  familiar  part  of  Mendelian  inheri- 
tance (Fig.  56).  Even  if  these  units  be  re- 


FIG.  56.  Diagram  showing  union  of  factors  in  fertilization 
and  their  segregation  in  the  formation  of  germ  cells.  With 
4  pairs  of  factors  (Aa,  Bb,  Cc,  Dd)  16  types  of  gametes  are 
possible  as  shown  in  the  two  series  of  small  circles  at  the 
right.  (From  Wilson.) 


PHENOMENA  OF   INHERITANCE 


263 


garded  as  physiological  processes  they  must  be 
associated  with  particular  structures,  since 
function  and  structure  are  inseparable  in  life 
processes.  What  are  these  units  in  terms  of 
cell  structures  and  where  are  they  located  in 
the  cell? 

We  have  in  the  chromosomes,  as  Wilson 
especially  has  emphasized,  an  apparatus  which 
fulfils  all  the  requirements  of  carriers  of 
Mendelian  factors  (Fig.  57).  Both  factors 


AbcD 

Sonatic    Division.  •"-*-  Red™""*  Gem    Cell. 

Division  Simplex    Groups 

Duplex    Grout* 

FIG.  57.  Diagram  of  germ  cells  corresponding  to  Fig.  56, 
showing  the  union  of  maternal  chromosomes  (ABCD)  and  pa- 
ternal ones  (abed)  in  fertilization,  their  distribution  in  cleav- 
age, their  union  into  4  pairs  (Aa,  Bb,  Cc,  Dd,)  in  synapsis 
and  the  separation  of  the  pairs  in  the  reduction  division. 
Only  2  of  the  16  possible  types  of  germ  cells  are  shown  at  the 
lower  right.  (From  Wilson.) 


264  HEREDITY  AND  ENVIRONMENT 

and  chromosomes  come  in  equal  numbers 
from  both  parents ;  both  maternal  and  paternal 
factors  and  chromosomes  pair  in  the  zygote 
and  separate  in  the  gamete  as  shown  in  Fig. 
56  and  Fig.  57;  and  so  far  as  is  known  the 
chromosomes  are  the  only  portions  of  the  germ 
cells  which  fulfil  these  conditions.  Further- 
more there  is  much  additional  evidence  that 
the  chromosomes  are  especially  concerned  in 
heredity,  as  was  pointed  out  in  the  last  lecture, 
and  it  is  not  reasonable  to  suppose  that  this 
remarkable  coincidence  between  the  distribu- 
tion of  Mendelian  factors  and  of  chromosomes 
is  without  significance. 

Of  course  Mendelian  factors  are  not  the  only 
factors  of  development  but  merely  the  differ- 
ential factors  which  cause,  for  example,  one 
guinea-pig  to  be  white  and  its  brother  to  be 
black.  Very  many  factors  are  involved  in  the 
production  of  white  or  black  color  but  there  is 
at  least  one  differential  factor  for  every  unit 
character  and  this  alone  is  the  Mendelian  fac- 
tor. Of  course  there  is  no  such  thing  as  a 
"sex-producing  chromosome,"  sex  being  the 
result  of  the  interaction  of  many  intrinsic  and 


PHENOMENA  OF   INHERITANCE  265 

extrinsic  causes.  The  X  chromosome  is  only 
one  factor  in  the  determination  of  sex,  but 
if  it  is  a  factor  which  differs  in  the  case  of 
the  two  sexes  it  is  a  "sex  determining  factor." 
There  are  many  parts  of  a  germ  cell,  all  of 
which  may  be  concerned  in  heredity  and  de- 
velopment, but  the  chromosomes  appear  to  be 
the  seat  of  the  differential  factors  for  Men- 
delian  characters. 

2.  Modifications  of  the  Principle  of  Domi- 
nance.— A  great  number  of  animal  and  plant 
hybrids  show  one  contrasting  character  com- 
pletely dominant  over  the  other  one  as  Mendel 
observed  in  the  case  of  his  peas.  But  in  a 
considerable  number  of  cases  this  dominance 
is  incomplete  or  imperfect.  When  white- 
flowered  strains  of  four  o'clocks  are  crossed 
with  red-flowered  ones  the  Fx  plants  bear 
neither  white  nor  red  flowers  but  pink  ones, 
and  the  F2  plants  bear  white,  red  and  pink 
flowers.  The  whites  and  reds  are  always 
homozygous,  the  pinks  heterozygous;  pure 
white  and  pure  red  are  produced  only  when 
their  factors  are  duplex  (WW),  (RR) ;  when 
they  are  simplex  (WR)  pink  is  produced.  In 


266  HEREDITY  AND  ENVIRONMENT 

this  case  red  is  not  completely  dominant  over 
white,  but  the  hybrid  is  more  or  less  interme- 
diate between  the  two  parents  (Fig.  50). 

It  has  long  been  known  that  the  breed  of 
fowls  called  Blue  Andalusian  does  not  breed 
true,  but  in  each  generation  produces  a  certain 
number  of  blacks  and  whites  as  well  as  blues. 
Bateson  found  that  the  blues  are  really  hybrids 
between  blacks  and  whites  in  which  neither  of 
the  latter  is  completely  dominant.  Black  and 
white  appear  only  when  they  are  pure  (homo- 
zygous),  blue  only  when  both  black  and  white 
are  present  (heterozygous). 

Again  a  cross  of  red  and  white  cattle  pro- 
duces roan  offspring,  but  the  latter  when  in- 
terbred give  rise  to  reds,  roans  and  whites  in 
the  proportion  of  1 :2 :1,  showing  that  the  roans 
are  heterozygotes  in  which  red  is  not  com- 
pletely dominant  over  white,  while  the  reds 
and  whites  are  homozygotes  and  consequently 
breed  true. 

Lang  found  that  when  snails  with  uniformly 
colored  shells  were  crossed  with  snails  having 
bands  of  color  on  t«he  shells  the  hybrids  were 
faintly  banded,  thus  being  more  or  less  inter- 


PHENOMENA  OF  INHERITANCE  267 

mediate  between  the  two  parents;  but  when 
these  hybrids  were  interbred  they  produced 
banded,  faintly  banded  and  uniformly  colored 
snails  in  the  ratio  of  1:2:1,  thus  proving  that 
Mendelian  segregation  takes  place  in  the  F2 
generation,  and  that  dominance  is  incomplete 
in  the  heterozygotes.  Many  other  similar 
cases  of  incomplete  dominance  are  known. 

Sometimes  dominance  is  incomplete  in  early 
stages  of  development  but  becomes  complete 
in  adult  stages.  Davenport  found  that  when 
pure  white  and  pure  black  Leghorn  fowls  are 
crossed  the  chicks  are  speckled  white  and 
black,  but  in  the  adult  fowl  dominance  is  com- 
plete and  the  plumage  is  black.  Similar  con- 
ditions of  delayed  dominance  are  well  known 
in  the  color  of  the  hair  and  eyes  of  children, 
though  dominance  may  become  complete  when 
they  have  reached  adult  life. 

In  a  few  instances  a  character  may  be  domi- 
nant at  one  time  and  recessive  at  another. 
Thus  Davenport  found  that  an  extra  toe  in 
fowls  is  dominant  under  certain  circumstances 
and  recessive  under  others.  Tennent  found 
that  characters  which  are  usually  dominant  in 


268  HEREDITY  AND  ENVIRONMENT 

hybrid  echinoderms  may  be  made  recessive  if 
the  chemical  or  physical  nature  of  the  sea  water 
is  changed.  Such  cases  seem  to  show  that 
dominance  may  depend  sometimes  upon  en- 
vironmental conditions,  sometimes  upon  a  par- 
ticular combination  of  hereditary  units. 

Sex  and  Sex-Limited  Inheritance 

Sex  and  sex-limited  inheritance  may  be  con- 
sidered here  since  they  involve  questions  of 
dominance,  certain  characters  remaining  un- 
developed in  one  sex  which  are  fully  developed 
in  the  other.  There  is  good  evidence,  as  was 
shown  in  the  last  lecture,  that  sex  is  a  Mendel- 
ian  character,  in  which  the  female  has  a  double 
dose  of  the  determiner  for  sex,  whereas  the 
male  has  only  a  single  dose.  Consequently  in 
the  formation  of  the  gametes  every  egg  re- 
ceives one  sex-determiner,  while  only  one-half 
of  the  spermatozoa  receive  such  a  determiner, 
the  other  half  of  them  being  without  it.  If 
then,  an  egg  is  fertilized  by  a  sperm  without 
one  of  these  determiners,  a  male  results;  but 
if  an  egg  is  fertilized  by  a  sperm  with  one  of 
these  determiners,  a  female  is  produced.  This 


PHENOMENA  OF  INHERITANCE 


is  graphically  represented  in  Fig.  58  in 
which  X  represents  the  sex-determiner,  which 
is  duplex  in  the  female  and  simplex  in  the 
male,  and  the  chance  unions  of  male  and  fe- 
male gametes  yield  females  (XX)  and  males 
(XO)  in  equal  numbers. 
In  either  sex  many  secondary  sexual  charac- 


Gametes 


Fi 


FIG.  58.  Diagram  showing  sex  as  a  Mendelian  character, 
the  female  being  homozygous,  the  male  heterozygous  for  sex. 
The  female  forms  gametes  all  of  which  contain  the  X 
chromosome;  the  male  forms  two  sorts  of  gametes  one-half 
of  which  contain  the  X  chromosome  and  the  other  half  lack 
it.  All  possible  combinations  of  these  gametes  give  a  2:2  or 
1:1  ratio  of  females  to  males. 


270  HEREDITY  AND  ENVIRONMENT 

ters  of  the  other  sex  are  present  during  de- 
velopment, and  traces  of  these  may  persist  in 
the  adult;  but  one  set  of  these  characters  de- 
velops in  the  male  and  another  in  the  female,  so 
that  they  may  be  called  sex-limited.  The  de- 
velopment of  the  secondary  sex  characters  is 
usually  determined  by  the  ovaries  or  testes, 
which  are  the  primary  sex  characters,  though 
in  some  instances  they  may  develop  in  animals 
which  have  lost  their  ovaries  or  testes,  but  in 
the  last  analysis  both  primary  and  secondary 
sex  characters  are  dependent  upon  the  sex- 
determiner.  Sex  and  sex-limited  inheritance 
are  only  special  cases  of  Mendelian  inheritance 
in  which  conditions  of  dominance  differ  in  the 
two  sexes,  depending  upon  whether  the  factor 
for  sex  is  duplex  or  simplex. 

Sex-Linked  Inheritance 

In  this  connection  we  may  consider  another 
class  of  characters,  which  are  linked  with  sex 
but  are  in  no  wise  connected  with  sexual  repro- 
duction. Such  characters  are  not  necessarily 
limited  to  one  sex  or  the  other,  as  are  many 
primary  and  secondary  sexual  characters,  but 


PHENOMENA   OF  INHERITANCE 


271 


they  may  appear  in  either  sex  though  they  are 
usually  transmitted  from  fathers  to  daughters, 
or  from  mothers  to  sons  ("criss-cross"  inheri- 
tance) in  exactly  the  way  in  which  the  sex 
chromosomes  (X)  are  transmitted.  Morgan 
has  therefore  concluded  that  the  factors  for 


Flies 


Chromosomes 


XX  X^  X©  W  F* 

$          ?         c?        c? 


FIG.  59.  Sex-linked  inheritance  of  white  and  red  eyes  in 
Drosophila.  Parents,  white-eyed  male  and  red-eyed  female; 
Fj,  red-eyed  males  and  females ;  F^  red-eyed  females  and  equal 
numbers  of  red-eyed  and  white-eyed  males.  The  distribution 
of  sex  chromosomes  is  shown  to  right  of  flies;  X  carries  the 
factor  for  red  eyes,  ^  tne  factor  for  white  eyes,  ©  stands  for 
absence  of  X-  (After  Morgan.) 


272 


HEREDITY  AND  ENVIRONMENT 


these  characters  are  carried  by  the  sex  chromo- 
somes and  has  named  them  sex-linked  charac- 
ters. In  the  fruit  fly,  Drosophila,,  he  has 
discovered  more  than  twenty-five  such  charac- 
ters, applying  to  the  color  of  the  eyes  and  of 
the  body,  to  the  length  of  the  wings,  etc.  A 
typical  case  is  shown  in  Figs.  59  and  60.  The 
eye  color  of  this  fly  is  normally  red,  but  muta- 


Flies 


Chromosomes 


Parents 


_ 

Gametes 


FIG.  60.  Reciprocal  cross  of  Fig.  61.  Parents,  white-eyed 
$  and  red-eyed  ^;  F,,  red-eyed  $  and  white-eyed  ^  ("criss- 
cross inheritance" ) ;  F2,  equal  numbers  of  red-eyed  5  and  <$ 
and  white-eyed  5  and  g.  The  distribution  of  the  sex  chromo- 
somes is  shown  on  the  right,  as  in  Fig.  59. 


PHENOMENA   OF  INHERITANCE  273 

tions  have  arisen  in  which  the  eye  is  white. 
Such  a  mutation  always  appears  in  males, 
though  it  may  later  be  transferred  to  females, 
as  we  shall  see.  If  now  a  white-eyed  male 
and  a  red-eyed  female  are  crossed  all  the  Fx  s 
are  red-eyed,  but  if  these  FI  s  are  interbred  all 
the  females  of  F2  have  red  eyes  while  half  of 
the  males  have  red  eyes  and  the  other  half 
have  white  eyes  (Fig.  59) .  On  the  other  hand 
if  one  of  the  FI  females  of  this  cross  is  bred 
with  a  white-eyed  male  half  of  the  females  of 
F2  are  red-eyed  and  half  are  white-eyed,  and 
half  of  the  males  are  red-eyed  and  half  are 
white-eyed. 

If  now  one  of  these  white-eyed  females  is 
bred  with  a  red-eyed  male  all  the  females  of 
the  F!  generation  are  red-eyed  and  all  the 
males  white-eyed  ("criss-cross"  inheritance) 
and  if  these  are  interbred  there  are  produced 
in  the  F2  generation  equal  numbers  of  red- 
eyed  and  white-eyed  males  and  females 
(Fig.  60). 

The  distribution  of  the  maternal  and  pa- 
ternal sex  chromosomes  (X)  exactly  parallels 
this  distribution  of  this  sex-linked  character, 


274  HEREDITY  AND  ENVIRONMENT 

as  is  shown  in  the  right  half  both  of  Fig.  59 
and  of  Fig.  60,  and  this  suggests  that  the 
differential  factors  for  these  characters  are  car- 
ried in  these  chromosomes. 

By  a  series  of  ingenious  experiments  Foot 
and  Strobell  have  shown  recently  that  the  dif- 
ferential factors  for  certain  sex-limited  char- 
acters in  insects,  that  is  characters  which  are 
limited  to  one  sex,  are  not  contained  in  the 
"sex  chromosomes,"  and  they  argue  that  the 
differential  factors  for  sex  and  for  sex-linked 
characters  cannot  be  located  in  these  chrom- 
osomes. Morgan  does  not  admit  the  validity 
of  their  conclusions,  but  it  must  be  admitted 
that  the  evidence  that  particular  determiners 
can  be  located  in  particular  chromosomes  is 
not  entirely  conclusive,  and  this  question  may 
for  the  present  be  regarded  as  an  open  one. 
However  there  is  good  evidence  that  the  fac- 
tors for  the  determination  of  sex  and  of  sex- 
linked  characters  are  distributed  in  the  same 
way  as  the  "sex  chromosomes"  are,  and  it 
would  be  a  surprising  thing  if  these  two  phe- 
nomena should  be  found  not  to  be  related 
causally. 


PHENOMENA  OF  INHERITANCE  275 

Another  case  of  sex-linked  inheritance  is 
found  in  an  abnormal  condition  in  man  known 
as  haemophilia,  which  is  characterized  by  a  de- 
ficiency in  the  clotting  power  of  the  blood,  and 
consequently  by  excessive  bleeding  after  in- 
jury. "Bleeders"  are  almost  always  males, 
though  the  defect  is  always  transmitted  to  a 
son  from  his  mother  who  does  not  usually  show 
the  defect  because  it  appears  in  females  only 
when  both  parents  were  affected.  The  man- 
ner of  inheritance  of  this  character  is  exactly 
similar  to  the  inheritance  of  white  eyes  in 
Drosophila  and  is  in  all  probability  due  to 
similar  causes. 

One  of  the  most  striking  cases  of  sex-linked 
inheritance  is  that  form  of  color  blindness 
known  as  Daltonism,  in  which  the  affected 
person  is  unable  to  distinguish  between  red 
and  green.  It  is  known  that  males  are  more 
frequently  affected  than  females,  and  that 
color  blindness  is  in  some  way  associated  with 
sex.  It  requires  two  determiners  for  color 
blindness,  one  from  the  father,  the  other  from 
the  mother,  to  produce  a  color  blind  female, 
whereas  only  a  single  determiner  is  necessary 


276  HEREDITY  AND  ENVIRONMENT 


to  produce  a  color  blind  male,  just  as  is  true 
of  sex.  The  accompanying  diagrams  (Figs. 
61,  62)  illustrate  the  method  of  inheritance  of 
color  blindness.  As  in  the  previous  diagrams 
X  represents  the  sex-determiner,  ©  its  absence, 
and  X  the  sex-determiner  which  carries  the 
factor  for  color  blindness. 

It  will  be  seen  that  a  color  blind  father  and  a 


Chromosomes 
I©  ^^      Parents 

Gametes 

Fi 
Gametes 

rvo    c?j 

«c>  "^    ^ 


FIG.  61.  Diagram  of  inheritance  of  color  blindness  through 
the  male.  A  color  blind  male  (here  black)  transmits  his 
defect  to  his  grandsons  only.  The  corresponding  distribution 
of  the  sex  chromosomes  is  shown  on  the  right,  the  one  carry- 
ing the  factor  for  color  blindness  being  black.  (After 
Morgan.) 


PHENOMENA  OF  INHERITANCE  277 

normal  mother  have  only  normal  children, 
but  the  father  transmits  to  his  daughters 
and  not  to  his  sons  the  sex-determiner  which 
carries  the  factor  for  color  blindness.  But 
since  color  blindness  does  not  develop  in  fe- 
males unless  it  is  duplex  (i.e.  comes  from  both 
father  and  mother)  whereas  it  develops  in 
males  if  it  is  simplex  (i.e.  comes  from  either 
parent)  all  the  daughters  of  a  color  blind 

Chromosomes 
W/ffo  VV 

&y  A  A       Parents 

c?  ? 

*, 

'  Gametes 

Fi 
Gametes 

<0><»S  <S.  <£,    M  XX  19  X9  v* 


FIG.  62.  Diagram  of  inheritance  of  color  blindness  through 
the  female.  A  color  blind  female  transmits  her  defect  to  all 
her  sons,  to  half  of  her  granddaughters  and  to  half  of  her 
grandsons.  Corresponding  distribution  of  sex  chromosomes  on 
right.  (After  Morgan.) 


278  HEREDITY  AND  ENVIRONMENT 

father  and  normal  mother  will  appear  normal 
although  carrying  one  determiner  for  color 
blindness,  while  all  the  sons  will  be  normal  be- 
cause they  carry  no  determiner  for  color  blind- 
ness. But  these  daughters  transmit  to  one-half 
of  their  children  the  single  determiner  for  color 
blindness  and  if  any  of  those  receiving  this  de- 
terminer are  males  they  will  be  color  blind. 
Consequently  we  have  the  curious  phenome- 
non of  simplex  color  blindness  appearing  only 
in  males  and  being  transmitted  to  them  only 
through  apparently  normal  females. 

On  the  other  hand  if  a  female  is  color  blind 
she  has  inherited  it  from  both  father  and 
mother,  i.e.  the  character  in  her  is  duplex,  and 
in  all  of  her  children  by  a  normal  male  the 
character  will  be  simplex;  accordingly  all  of 
her  sons  will  be  color  blind  and  all  of  her 
daughters  will  be  normal,  though  carrying  the 
simplex  determiner  for  color  blindness. 


In  all  cases  dominance  means  merely  the 
development  in  offspring  of  certain  characters 
of  one  parent,  while  contrasting  characters  of 


PHENOMENA  OF  INHERITANCE  279 

the  other  parent  remain  undeveloped.  The 
appearance  of  any  developed  character  in  an 
organism  depends  upon  many  complicated 
reactions  of  germinal  units  to  one  another  and 
to  the  environment.  Under  certain  conditions 
of  the  germ  or  of  the  environment  some  char- 
acters may  develop  in  hybrids  to  the  exclusion 
of  their  opposites  whereas  under  other  condi- 
tions these  results  may  be  reversed  or  the  char- 
acters may  be  intermediate.  The  principle  of 
dominance  is  not  a  fundamental  part  of  Men- 
delian  inheritance.  Even  when  the  characters 
of  hybrids  are  intermediate  between  those  of 
their  parents,  if  the  parental  types  reappear 
in  the  F2  generation  we  may  be  certain  that 
we  are  dealing  with  cases  of  Mendelian 
inheritance. 

3.  The  Principle  of  Segregation. — The  in- 
dividuality of  inheritance  units,  and  their  seg- 
regation or  separation  in  the  sex  cells  and 
recombination  in  the  zygote  are  fundamental 
principles  of  the  Mendelian  doctrine.  Indeed 
the  evidence  for  the  individuality  and  contin- 
uity of  inheritance  units  is  based  entirely  upon 
such  segregation  and  recombination,  so  that 


280  HEREDITY  AND  ENVIRONMENT 

the  entire  Mendelian  theory  may  be  said  to 
rest  upon  the  principle  of  segregation.  If 
there  are  cases  in  which  such  segregation  does 
not  take  place  they  belong  to  other  forms  of 
inheritance  than  the  Mendelian ;  if  segregation 
occurs  in  every  instance  there  is  no  other  type 
of  inheritance  than  that  discovered  by  Mendel. 
Are  there  cases  which  do  not  segregate  ac- 
cording to  Mendelian  expectation? 

When  the  Mendelian  theory  was  new  it  was 
generally  supposed  that  there  were  forms  of 
inheritance  which  differed  materially  from  the 
Mendelian  type;  indeed  it  was  supposed  that 
the  latter  was  one  of  the  less  common  forms 
of  heredity  and  that  blending  of  parental 
traits  and  not  segregation  was  the  rule.  All 
cases  in  which  the  characters  of  the  parents 
appeared  to  blend  in  the  offspring  or  in  which 
there  was  not  a  clear  segregation  of  the  par- 
ental types  in  the  F2  generation  or  in  which 
the  ratio  for  a  monohybrid  differed  from  the 
well  known  3  to  1  ratio  were  supposed  to  be 
non-Mendelian. 

However  further  work  has  shown  that  some 
of  these  are  really  Mendelian.  Sometimes 


.    PHENOMENA  OF  INHERITANCE  281 

offspring  are  intermediate  between  their 
parents  owing  to  incompleteness  of  dominance, 
rather  than  to  incompleteness  of  segregation; 
in  such  cases  the  parental  types  reappear  in 
the  F2  generation  as  in  the  cross  between  red 
and  white  four  o'clocks.  Sometimes  depar- 
tures from  the  3  to  1  ratio  are  caused  by  the 
fact  that  two  or  more  factors  of  the  same  sort 
are  involved  in  the  production  of  a  single  char- 
acter. Nilsson-Ehle  found  that  when  oats 
with  black  glumes  were  crossed  with  varieties 
having  white  glumes  the  ratio  of  3  white  to 
1  black  was  usually  found  in  the  second  gen- 
eration; but  one  variety  of  black  oats  when 
crossed  with  white  gave  in  the  second  gener- 
ation approximately  15  blacks  to  1  white 
which  is  the  dihybrid  ratio.  From  this  and 
other  evidence  he  concludes  that  in  this  va- 
riety of  oats  two  hereditarily  separable  fac- 
tors are  involved  in  the  production  of  black. 
In  crosses  between  red-grained  and  white- 
grained  wheat  he  usually  got  in  the  second 
generation  the  monohybrid  ratio  of  3  red  to  1 
white,  but  three  strains  gave  the  dihybrid  ratio 
of  15  to  1  and  two  gave  the  trihybrid  ratio  of 


282  HEREDITY  AND  ENVIRONMENT     . 

63  to  1.  Consequently  he  concludes  that  while 
the  red  color  of  wheat  grains  is  usually  due 
to  one  factor  for  red,  it  may  in  some  cases  be 
due  to  two  or  even  three  factors;  notable  de- 
partures from  expected  ratios  may  thus  be 
explained. 


Blending  Inheritance 

But  the  most  serious  objections  whicn  van 
be  presented  against  the  universality  of  the 
Mendelian  doctrine  are  found  in  phenomena 
of  "blending"  inheritance.  In  some  instances 
contrasting  characters  of  parents  appear  to 
blend  in  offspring  and  even  in  the  F2  and  in 
subsequent  generations  the  descendants  remain 
more  or  less  intermediate  between  the  par- 
ents. One  of  the  best  known  illustrations  of 
this  is  found  in  the  skin  color  of  the  mulatto 
which  is  intermediate  between  the  white  parent 
and  the  black  one,  and  even  in  the  F2  and  in 
subsequent  generations  mulattoes  do  not  usu- 
ally, if  ever,  produce  pure  white  or  pure  black 
children,  though  the  children  of  mulattoes  show 
considerable  variation  in  color.  Here  there  is 


PHENOMENA  OF  INHERITANCE  283 

an  apparent  failure  of  the  Mendelian  princi- 
ple of  segregation. 

But  white  skin  is  not  really  white  nor  is 
black  skin  ever  perfectly  black.  Davenport 
has  shown  that  there  is  a  mixture  of  black, 
yellow  and  red  pigment  in  both  white  and 
black  skins,  though  the  amount  of  each  of 
these  pigments  varies  greatly  in  negroes  and 
whites.  The  relative  amounts  of  these  pig- 
ments in  any  given  case  may  be  determined  by 
means  of  a  rotating  color  disk.  A  white  person 
may  have  a  skin  color  composed  of  black  (b) 
8  per  cent.,  yellow  (y)  9  per  cent.,  red  (r)  50 
per  cent.,  and  absence  of  pigment  or  white  (w) 
33  per  cent.  On  the  other  hand  a  very  black 
negro  may  have  b  68  per  cent.,  y  2  per  cent., 
r  26  per  cent.,  w  4  per  cent.  The  nine  children 
of  two  mulattoes,  the  father  having  13  per  cent, 
of  black  and  the  mother  45  per  cent.,  ranged  all 
the  way  from  46  per  cent,  to  6  per  cent,  of 
black,  the  latter  so  far  as  skin  color  is  con- 
cerned being  virtually  white.  On  the  other 
hand  where  both  parents  have  about  the  same 
degree  of  pigmentation  the  children  are  more 
nearly  uniform  in  color ;  thus  seven  children  of 


284  HEREDITY  AND  ENVIRONMENT 

two  mulattoes,  the  father  having  36  per  cent, 
and  the  mother  30  per  cent,  of  black,  ranged 
only  from  27  per  cent,  to  39  per  cent,  of  black. 

Such  variations  in  color  in  the  F2  and  in 
subsequent  generations  are  exactly  what  one 
would  expect  in  a  Mendelian  character  in 
which  more  than  one  factor  is  involved,  as  for 
example  in  the  case  of  the  color  of  the  sweet 
peas  shown  in  Fig.  55.  Davenport,  who  has 
made  an  extensive  study  of  this  case,  con- 
cludes that  "there  are  two  double  factors 
(A A,  BE)  for  black  pigmentation  in  the  full 
blooded  negro  of  the  west  coast  of  Africa,  and 
these  are  separably  inheritable."  These  fac- 
tors are  lacking  in  white  persons  (this  being 
indicated  by  the  formula  aa,  bb).  Since  the 
germ  cells  carry  only  single  factors  and  not 
double  ones  the  cross  between  negro  and  white 
would  have  only  one  set  of  these  factors  for 
black  color,  as  shown  by  the  formula  AB  x  ab 
=  A  Bab;  hence  the  color  of  the  FI  generation 
is  intermediate  between  that  of  the  two  par- 
ents. In  the  F2  generation  there  should  be  a 
variety  of  colors  ranging  all  the  way  from 
white  to  black,  though  pure  white  or  pure  black 


PHENOMENA  OF   INHERITANCE  285 

would  be  expected  in  only  a  small  proportion 
of  the  offspring.  As  a  matter  of  fact  it  is 
known  that  the  children  of  mulattoes  vary 
considerably  in  color,  and  in  some  cases  a  child 
may  be  darker  or  lighter  than  either  parent, 
which  would  indicate  that  segregation  does 
actually  occur.  It  is  very  probable  that  this 
classical  case  of  "blending"  inheritance  is  really 
Mendelian  inheritance  in  which  two  or  more 
factors  for  skin  color  are  involved. 

Similar  "blending"  inheritance  is  found  in 
certain  other  cases  where  the  parents  differ  in 
form  or  size.  Thus  Castle  found  that  when 
long-eared  rabbits  were  crossed  with  short- 
eared  ones  the  offspring  have  ears  of  inter- 
mediate length,  and  in  all  subsequent  gener- 
ations the  ear  length  remained  intermediate 
between  that  of  the  parents.  He  found  the 
same  thing  true  of  length  and  breadth  of  the 
skull  (Fig.  63)  and  of  the  size  of  other  portions 
of  the  skeleton,  and  he  concluded  that  such 
quantitative  characters  are  not  inherited  in 
Mendelian  fashion. 

Quite  recently  MacDowell,  working  on  the 
inheritance  of  size  in  rabbits,  concludes  that 


286  HEREDITY  AND  ENVIRONMENT 

this  character,  as  well  as  other  quantitative  dif- 
ferences between  parents  which  appear  to 
blend  in  the  offspring,  such  as  Castle's  case  of 
ear  length  in  rabbits,  is  not  due  to  a  single  fac- 
tor, as  in  the  case  of  Mendel's  tall  and  dwarf 
peas,  but  to  several  factors.  Consequently  in 
the  formation  of  the  germ  cells  there  is  not  a 
clean  segregation  of  all  the  factors  for  tallness 


FIG.  63.  Blending  inheritance  of  size  in  rabbits;  the  skulls 
of  two  parents  are  shown  in  1  and  3,  of  their  intermediate 
offspring  in  2.  (From  Castle.) 


PHENOMENA  OF  INHERITANCE  287 

or  large  size  or  long  ears  in  half  the  germ  cells 
and  their  total  absence  in  the  other  half  of 
those  cells,  but  some  of  these  factors  go  into 
certain  cells  and  others  into  others,  as  in  the 
case  of  dihybrids,  trihybrids  or  polyhybrids. 
As1  a  result  offspring  appear  more  or  less  in- 
termediate in  size  between  their  parents. 

Thus  it  is  possible  to  explain  even  "blend- 
ing" inheritance  as  due  not  to  the  real  fusion 
or  blending  of  inheritance  factors  but  to  vary- 
ing combinations  of  numerous  or  multiple  fac- 
tors, according  to  the  Mendelian  rules.  The 
Mendelian  principle  of  segregation  has  been 
found  to  be  of  such  general  occurrence  that 
there  is  a  strong  inclination  among  Mendel- 
ians  of  the  stricter  sort  to  make  it  universal, 
and  to  explain  all  cases  of  "blending"  inheri- 
tance as  due  to  incomplete  dominance  and  to 
multiple  factors.  Whether  or  not  such  at- 
tempts may  prove  completely  successful  it  is 
still  too  soon  to  say. 


288  HEREDITY  AND  ENVIRONMENT 

III.  MENDELIAN  INHERITANCE  IN  MAN 

The  study  of  inheritance  in  man  must  al- 
ways be  less  satisfactory  and  the  results  less 
secure  than  in  the  case  of  lower  animals  and 
for  the  following  reasons:  In  the  first  place 
there  are  no  "pure  lines"  but  the  most  compli- 
cated intermixture  of  different  lines.  In  the 
second  place  experiments  are  out  of  the  ques- 
tion and  one  must  rely  upon  observation  and 


FIG.  64.  Blending  inheritance  of  skin  color  in  negro — 
white  cross  with  dominance  of  kinky  hair  over  straight  hair. 
(From  Bond.) 


PHENOMENA  OF  INHERITANCE  289 


FIG.  65.     Papuan    family    of   New   Guinea   with   one    albino 
child.     (From  Pearson-Nettleship-Usher.) 


290  HEREDITY  AND  ENVIRONMENT 

statistics.  In  the  third  place  man  is  a  slow 
breeding  animal;  there  have  been  less  than 
sixty  generations  of  men  since  the  beginning  of 
the  Christian  era,  whereas  Jennings  gets  as 
many  generations  of  Paramecium  within  two 
months  and  Morgan  almost  as  many  genera- 
tions of  Drosophila  within  two  years.  Finally 
the  number  of  offspring  are  so  few  in  human 
families  that  it  is  difficult  to  determine  what  all 
the  hereditary  possibilities  of  a  family  may  be. 
Bearing  in  mind  these  serious  handicaps  to  an 
exact  study  of  inheritance  it  is  not  surprising 
that  the  method  of  inheritance  of  many  human 
characters  is  still  uncertain. 

Davenport  and  Plate  have  catalogued  more 
than  sixty  human  traits  which  seem  to  be  in- 
herited in  Mendelian  fashion.  About  fifty  of 
these  represent  pathological  or  teratological 
conditions  while  only  a  relatively  small  num- 
ber are  normal  characters.  This  does  not  sig- 
nify that  the  method  of  inheritance  differs  in 
the  case  of  normal  and  abnormal  characters, 
but  rather  that  abnormal  characters  are  more 
striking,  more  easily  followed  from  generation 
to  generation,  and  consequently  statistics  are 


PHENOMENA  OF  INHERITANCE 


more  complete  with  regard  to  them  than  in  the 
case  of  normal  characters.  In  many  cases  sta- 
tistics are  not  sufficiently  complete  to  deter- 
mine with  certainty  whether  the  character  in 
question  is  dominant  or  recessive,  and  it  must 
be  understood  that  in  some  instances  the  classi- 
fication in  this  respect  is  tentative.  A  partial 
list  of  these  characters  is  given  herewith: 

MENDELIAN  INHERITANCE  IN  MAN 

NORMAL    CHARACTERS 

Dominant  Recessive 


Hair: 

Curly 

Dark 
Eye  Color: 

Brown 
Skin  Color: 

Dark' 

'  Normal    Pigmentation 
Countenance: 

Hapsburg  Type  (Thick 
lower  lip  and  promi- 
nent chin) 

German   Type 
Temperament: 

Nervous 
Intellectual  Capacity : 

Average 

Average 


Straight   (Fig.  64) 
Light  to  red 

Blue 

Light 

Albinism  (Figs.  65,  66) 

Normal 


Jewish  Type 
Phlegmatic 

Very  great 
Very  small 


292  HEREDITY  AND  ENVIRONMENT 


FIG.  66.     European  family  with  three  albino  and  four  nor- 
mal   children.     (From    Pearson-Nettleship- Usher.) 


B  A 

FIG.  67.    X-ray  picture,  A  of  a  normal,  B  of  a  short-fingered 
(brachydactyl)  hand.     (From  Bateson.) 


PHENOMENA  OF  INHERITANCE 


293 


MENDELIAN  INHERITANCE  IN  MAN  (Continued) 

TERATOLOGICAL   AND   PATHOLOGICAL    CHARACTERS 


Dominant 
General  Size: 

Achondroplasy    (Dwarfs 

with  short  stout  limbs 

but    with    bodies    and 

heads  of  normal  size) 

Normal   Size 


Hands  and  Feet: 

Brachydactyly  (Short 
fingers  and  toes) 

Syndactyly  (Webbed 
fingers  and  toes) 

Polydactyly      (Supernu- 
merary digits) 
Skin: 

Keratosis  (Thickening 
of  Epidermis) 

Epidermolysis  (Exces- 
sive formation  of  blis- 
ters) 

Hypotrichosis         (Hair- 
lessness         associated 
with  lack  of  teeth) 
Kidneys : 

Diabetes   insipidus 

Diabetes  mellitus 

Normal 


Recessive 


Normal   (Fig.  68) 


True  Dwarfs  (With  all 
parts  of  the  body  re- 
duced in  proportion 
Fig.  68) 

Normal  (Fig.  67) 

Normal 

Normal 


Normal 


Normal 


Normal 


Normal 
Normal 

Alkaptonuria          (Urine 
dark   after   oxidation) 


PHENOMENA  OF  INHERITANCE 


295 


MENDELIAN  INHERITANCE  IN  MAN   (Continued) 

TERATOLOGICAL     AND     PATHOLOGICAL     CHARACTERS 


Dominant 
Nervous  System: 
Normal  Condition 


Normal 


Normal 


Normal 


Normal 


Normal 


Huntington's  Chorea 
Muscular  Atrophy 
Eyes: 

Hereditary  Cataract 
Pigmentary      Degenera- 
tion of  Retina 


Recessive 

General    Neuropathy,    e.g. 
Hereditary  Epilepsy 
Hereditary  Feeble-mind- 

edness 

Hereditary  Insanity 
Hereditary    Alcoholism 
Hereditary    Criminality 
Hereditary   Hysteria 
Multiple  Sclerosis  (Dif- 
fuse   degeneration    of 
nerve  tissue) 
Friedrich's  Disease  (De- 
generation    of     upper 
part  of  spinal  cord) 
Meniere's  Disease  (Diz- 
ziness  and   roaring  in 
ears) 
Chorea         (St.         Vitus 

Dance) 

Thomsen's      Disease 
(Lack     of      muscular 
tone) 
Normal 
Normal 

Normal 
Normal 


296  HEREDITY  AND  ENVIRONMENT 

MENDELIAN  INHERITANCE  IN  MAN  (Continued) 

TERATOLOGICAL    AND    PATHOLOGICAL    CHARACTERS 

Dominant  Recessive 

Glaucoma  (Internal  pres-       Normal 
sure   and   swelling   of 
eyeball) 
Coloboma    (Open   suture          Normal 

in  iris) 

Displaced   Lens  Normal 

Ears: 

Normal  Deaf-mutism 

Normal  Otosclerosis    (Thickened 

tympanum  with  hard- 
ness of  hearing) 

SEX-LINKED    CHARACTERS 

Recessive  characters,  appearing  in  male  when  sim- 
plex, in  female  only  when  duplex. 

Normal  Gower's  Muscular  Atro- 

phy 

Normal  Haemophilia  (Slow  clot- 

ting of  blood) 

Normal  Color  Blindness  (Dalt- 

onism ;  inability  to  dis- 
tinguish red  from 
green) 

Normal  Night  Blindness  (Ina- 

bility to  see  by  faint 
light) 

Normal  Neuritis  Optica  (Pro- 

gressive atrophy  of 
optic  nerve) 


PHENOMENA  OF  INHERITANCE  297 

SUMMARY 

The  principles  of  heredity  established  by 
Mendel  are  almost  as  important  for  biology 
as  the  atomic  theory  of  Dalton  is  for  chem- 
istry. By  means  of  these  principles  par- 
ticular dissociations  and  recombinations  of 
characters  can  be  made  with  almost  the  same 
certainty  as  particular  dissociations  and  recom- 
binations of  atoms  can  be  made  in  chemical 
reactions.  By  means  of  these  principles  the 
hereditary  constitution  of  organisms  can  be 
analyzed  and  the  real  resemblances  and  differ- 
ences of  various  organisms  determined.  By 
means  of  these  principles  the  once  mysterious 
and  apparently  capricious  phenomena  of  pre- 
potency, atavism  and  reversion  find  a  satis- 
factory explanation. 

Before  the  establishment  of  Mendel's  prin- 
ciples heredity  was,  as  Balzac  said,  "a  maze 
in  which  science  loses  itself."  Much  still  re- 
mains to  be  discovered  about  inheritance,  but 
the  principles  of  Mendel  have  served  as  an 
Ariadne  thread  to  guide  science  through  this 
maze  of  apparent  contradictions  and  excep- 
tions in  which  it  was  formerly  lost. 


CHAPTER  IV 
INFLUENCE  OF  ENVIRONMENT 


CHAPTER  IV 


INFLUENCE  OF  ENVIRONMENT 

The  development  of  an  individual  or  the 
evolution  of  a  race  is  dependent  upon  the  in- 
teraction of  two  sets  of  factors  or  causes,  the 
intrinsic  and  the  extrinsic.  The  former  is  rep- 
resented by  the  organization  of  the  germinal 
protoplasm,  the  latter  by  all  other  conditions; 
the  former  is  known  as  heredity  or  constitu- 
tion, the  latter  as  environment  or  education; 
or  in  the  words  of  Galton,  these  two  sets  of 
factors  may  be  called  "nature"  and  "nurture." 
The  great  problem  of  development  is  the  un- 
raveling of  these  two  factors,  the  assignment 
of  its  true  value  to  each,  and  the  ultimate  con- 
trol of  development  so  far  as  this  may  be  pos- 
sible through  the  knowledge  thus  gained. 


301 


302  HEREDITY  AND  ENVIRONMENT 

A.  RELATIVE  IMPORTANCE  OF  HEREDITY 
AND  ENVIRONMENT 

The  distinction  between  these  two  factors  of 
development  is  generally  recognized  and  the 
question  of  the  relative  importance  of  the  two 
has  been  discussed  for  ages.  Which  is  the 
more  important,  constitution  or  environment? 
What  characteristics  are  due  to  nature  and 
what  to  nurture?  To  what  extent  is  man  the 
creature  of  heredity,  to  what  extent  the  pro- 
duct of  education?  The  old  question  "Which 
of  you  by  taking  thought  can  add  one  cubit 
to  his  stature,"  is  a  vital  question  to-day.  To 
what  extent  may  nature  be  modified  by  nur- 
ture ?  To  what  extent  may  education  make  up 
for  deficiencies  of  birth? 

1.  Formerly  very  great  emphasis  was 
placed  upon  influences  of  environment  in 
phylogeny  and  ontogeny.  From  the  earliest 
times  it  has  been  believed  that  species  might 
be  transmuted  by  environmental  changes  and 
that  even  life  itself  might  arise  from  lifeless 
matter  through  the  influence  of  favorable  ex- 
trinsic conditions.  If  environment  could  exert 


INFLUENCE  OF  ENVIRONMENT  303 

so  great  an  influence  on  the  origin  of  species 
or  even  of  life  itself  much  more  could  it  affect 
the  process  of  development  of  the  individual. 
It  is  still  popularly  supposed  that  complexion 
is  dependent  upon  the  intensity  of  light,  and 
stature  upon  the  quantity  and  quality  of  food, 
that  sex  is  determined  by  food  or  temperature, 
mentality  by  education,  and  that  in  general 
individual  peculiarities  are  due  to  environ- 
mental differences. 

Many  philosophers  of  the  seventeenth  and 
eighteenth  centuries  taught  that  man  was  the 
product  of  environment  and  education  and  that 
all  men  were  born  equal  and  later  became  un- 
equal through  unequal  opportunities.  Des- 
cartes begins  his  famous  "Discourse  on 
Method"  with  these  words: 

"Good  sense  is,  of  all  things  among  men,  the  most 
equally  distributed  .  .  .  The  diversity  of  our  opin- 
ions does  not  arise  from  some  being  endowed  with  a 
larger  share  of  Reason  than  others,  but  solely  from 
this,  that  we  conduct  our  thoughts  along  different 
ways,  and  do  not  fix  our  attention  on  the  same 
objects." 

The  Declaration  of  Independence  merely  re- 
flected the  spirit  of  the  age  in  which  it  was 


304  HEREDITY  AND  ENVIRONMENT 

written  when  it  held  this  truth  to  be  self  evi- 
dent, "that  all  men  are  created  free  and  equal." 
The  equality  of  man  has  always  been  one  of 
the  foundation  stones  of  democracy.  Upon 
this  belief  in  the  natural  equality  of  all  men 
were  founded  systems  of  theology,  education 
and  government  which  hold  the  field  to  this 
day.  Upon  the  belief  that  men  are  made  by 
their  environment  and  training  rather  than  by 
heredity  are  founded  most  of  our  social  institu- 
tions with  their  commands  and  prohibitions, 
their  rewards  and  punishments,  their  charities 
and  corrections,  their  care  for  the  education 
and  environment  of  the  individual  and  their 
disregard  of  the  inheritance  of  the  race.  Civi- 
lization itself  means  only  good  environmental 
conditions,  and  the  advance  of  civilization 
means  only  improvement  of  environment. 

2.  On  the  other  hand  modern  studies  in 
genetics  are  emphasizing  the  immense,  the 
overwhelming  importance  of  heredity,  in  both 
phylogeny  and  ontogeny.  No  one  now  takes 
seriously  the  assertion  that  life  can  be  experi- 
mentally produced  at  the  present  time  from 
non-living  matter.  It  is  evident  that  the  arti- 


INFLUENCE  OF  ENVIRONMENT  305 

ficial  production  of  life  is  a  much  more  diffi- 
cult problem  than  was  once  supposed,  and  it 
may  be  an  insoluble  problem.  The  first  flush 
of  enthusiasm  over  experimental  methods  in 
biology  led  to  the  expectation  that  we  would 
soon  be  making  species  and  indeed  whole 
faunas  and  floras  by  the  process  of  experi- 
mental evolution,  but  the  results  of  one  or  two 
decades  of  such  experimental  work  have  been 
somewhat  disappointing.  Inherited  variations 
do  appear,  incipient  species  arise,  but  there  is 
very  little  evidence  to  show  that  they  appear 
in  response  to  environmental  changes  only. 
Belief  in  the  omnipotence  of  environment  in 
the  evolution  of  species  has  steadily  waned  in 
recent  years,  while  a  belief  in  the  intrinsic 
causes  of  evolution,  such  as  the  mutation 
theory  and  orthogenesis,  has  increased. 

In  ontogeny  also  the  environmental  or  ex- 
trinsic factors  of  development  have  been 
relegated  to  a  subordinate  place,  while  the  in- 
trinsic or  hereditary  factors  appear  more  im- 
portant than  ever.  The  old  view  that  men  are 
chiefly  the  product  of  environment  and  train- 
ing is  completely  reversed  by  recent  studies 


306  HEREDITY  AND  ENVIRONMENT 

of  heredity.  The  modifications  which  may  be 
produced  by  environment  and  education  are 
small  and  temporary  as  compared  with  those 
which  are  determined  by  heredity. 

3.  These  conclusions  are,  in  the  main,  well 
founded.  The  evidence  of  the  tremendous  im- 
portance of  heredity  is  so  complete  that  we 
may  rest  assured  that  thinking  men  will  never 
again  return  to  the  position  which  prevailed 
until  a  few  years  ago  regarding  the  all-impor- 
tance of  environment.  And  yet  there  is  danger 
of  going  too  far  in  the  opposite  direction. 
Neither  environment  nor  heredity  is  all-im- 
portant, but  both  are  necessary  to  develop- 
ment. The  germ  cells  with  all  their  inherent 
possibilities  would  forever  remain  germ  cells 
were  it  not  for  environmental  stimuli.  The 
realization  of  germinal  possibilities  is  depend- 
ent upon  the  responses  of  the  germ  to  en- 
vironmental stimuli,  and  although  heredity  is 
a  relatively  constant  factor  while  environment 
is  a  more  variable  one,  nevertheless  the  two 
are  indispensable  to  development.  Only  by 
experiment  can  the  relative  importance  of 
heredity  and  environment  in  development  be 


INFLUENCE  OF  ENVIRONMENT  SOT 

determined.  Extensive  experiments  have 
been  made  within  recent  years  on  developing 
animals  and  plants  in  order  to  discover  the 
factors  involved  in  development,  and  the  modi- 
fications which  may  thus  be  produced  are  very 
striking. 


B.  EXPERIMENTAL  MODIFICATION  OF 
DEVELOPMENT 

The  study  of  development  under  experi- 
mental conditions  has  given  rise  to  a  new 
branch  of  biology,  viz.,  experimental  embry- 
ology or  the  physiology  of  development.  By 
changes  in  environmental  conditions  notable 
modifications  may  be  produced  in  adult  or- 
ganisms, but  these  modifications  are  much 
greater  when  the  changed  environment  acts 
on  the  organism  during  the  period  of  its  de- 
velopment. 

I.  DEVELOPMENTAL  STIMULI 

It  is  by  no  means  easy  to  define  such  gen- 
eral terms  as  "environment,"  "stimulus,"  "re- 
sponse." In  its  common  use  "environment" 


308  HEREDITY  AND  ENVIRONMENT 

means  all  that  lies  outside  the  individual,  if  it  is 
defined  from  the  standpoint  of  the  entire  or- 
ganism ;  but  from  the  standpoint  of  an  organ  or 
cell  it  is  the  surrounding  organs,  cells  or  fluids 
of  the  body — the  latter  may  be  defined  as  in- 
ternal environment.  If  developmental  stimuli 
arise  outside  the  organism  they  are  plainly  ex- 
trinsic or  environmental,  but  if  they  arise 
within  the  organism  they  are  said  to  be  intrin- 
sic though  they  may  be  due  to  changes  in  the 
internal  environment. 

Stimuli  are  chiefly  energy  changes  of  a 
physical  or  chemical  nature.  A  stimulus  to 
which  an  adult  organism  responds  by  move- 
ments or  other  activities  may  call  forth  or  in- 
hibit developmental  responses  when  applied  to 
germ  cells  or  embryos. 

These  developmental  stimuli  may  be  classed 
as: 

1.  Physical  stimuli  including  the  following, 
(a)   mechanical,    (b)    thermal,    (c)    electrical, 
(d)  radiant,  (e)  light,  (f)  density  of  medium, 
(g)  gravity  and  centrifugal  force,  etc. 

2.  Chemical  stimuli  include  the  action  of 
(a)  substances  found  in  normal  development, 


INFLUENCE  OF  ENVIRONMENT  309 

such  as  oxygen,  carbonic  acid,  water,  food, 
secretions  of  ductless  glands  etc.,  and  (b)  sub- 
stances not  found  in  normal  development, 
such  as  various  salts,  acids,  alkalis,  alcohol, 
ether,  tobacco,  etc. 

3.  In  general  the  action  of  these  stimuli 
during  development  does  not  call  forth  a  per- 
fectly specific  and  definite  response  of  the  or- 
ganism ;  various  stimuli  may  produce  the  same 
result.  Thus  artificial  parthenogenesis  has 
been  produced  by  almost  every  stimulus  named, 
and  weakened  or  retarded  development  is  pro- 
duced by  many  different  stimuli. 

By  the  elimination  of  certain  of  these 
stimuli  which  are  normally  present  or  by  in- 
troducing stimuli  which  are  not  usually  pres- 
ent very  important  and  even  profound 
changes  in  development  may  be  produced.  In 
this  way  animals  have  been  formed  which  are 
turned  inside  out,  or  side  for  side,  or  in  which 
heads  or  nervous  systems  or  muscles  or  back- 
bones are  lacking,  or  in  which  the  various  or- 
gans are  not  found  in  normal  positions.  In 
this  way  dwarfs  and  giants  and  one-eyed 
monsters  as  well  as  all  sorts  of  double  and  par- 


310  HEREDITY  AND  ENVIRONMENT 

tial  embryos  have  been  formed.  In  general 
monstrous  and  defective  forms  of  development 
are  due  to  alterations  of  the  normal  environ- 
mental stimuli  rather  than  to  defective  he- 
reditary constitution. 

II.  DEVELOPMENTAL  RESPONSES 
The  character  of  developmental  responses 
to  stimuli  depends  primarily  upon  (a)  the  na- 
ture of  the  organism  and  (b)  the  stage  of 
development  at  which  the  stimulus  acts.  Modi- 
fications are  more  easily  produced  and  are 
more  profound  during  cell  division  than  during 
intervening  periods  and  at  early  stages  of  de- 
velopment than  at  later  ones. 

1.  Modifications  of  Germ  Cells  before  Ferti- 
lization.— It  has  been  found  by  many  investi- 
gators that  development  may  be  profoundly 
changed  by  influences  acting  upon  the  germ 
cells  before  fertilization.  In  general  environ- 
mental changes  acting  during  the  growth  of 
eggs  or  spermatozoa  and  especially  during 
their  maturation  may  produce  marked  changes 
in  development  and  even  in  heredity.  Tower 
has  found  that  unusual  conditions  of  temper- 
ature and  humidity  during  the  later  stages  of 


INFLUENCE  OF  ENVIRONMENT  311 

oogenesis  and  spermatogenesis  may  lead  to 
the  production  of  new  races  in  the  case  of  the 
potato  beetle  (Fig.  94).  I  have  found  that  a 
slight  increase  in  temperature  at  the  time  of 
nuclear  division  may  lead  to  abnormal 
separation  of  the  chromosomes  and  presum- 
ably to  a  change  in  hereditary  constitution; 
Gager  has  obtained  similar  results  following 
the  use  of  radium  on  plants.  MacDougall's 
experiments  on  plants  point  to  the  conclusion 
that  chemical  substances  may  influence  the 
ovules  so  as  to  change  the  hereditary  character 
of  the  plant.  Bardeen  and  the  Hertwigs  have 
shown  that  great  monstrosities  may  be  pro- 
duced if  X-rays,  radium  or  various  chemical 
substances  are  allowed  to  act  on  spermatozoa 
before  fertilization.  Stockard  subjected  adult 
male  and  female  guinea-pigs  to  the  fumes  of 
alcohol  for  some  time  before  breeding  them  and 
then  studied  the  effects  of  this  drug  on  their 
offspring.  He  finds  that  the  influence  of  alco- 
hol on  the  spermatozoa  is  as  deleterious  as 
when  acting  on  the  ova  and  that  it  produces 
sterility,  or  greatly  reduced  fertility,  a  great 
excess  of  still-births,  and  weak  and  sickly  off- 


312 


HEREDITY  AND  ENVIRONMENT 


spring  ( Fig.  69 ) .  Hoppe  believes  that  a  single 
drunken  debauch  may  so  injure  the  germ  cells 
of  man  as  to  produce  abnormal  and  defective 


FIG.  69.  Dwarfed  guinea-pigs  on  the  left  and  normal  ones 
on  the  right.  All  are  of  approximately  the  same  age  though 
the  normal  ones  are  nearly  twice  the  weight  of  the  dwarfs. 
The  normals  came  from  normal  parents,  the  dwarfs  from  a 
normal  mother  and  an  alcoholic  father;  the  dwarfing  has 
therefore  been  produced  by  the  influence  of  alcohol  on  the 
spermatozoa.  (From  Stockard.) 


INFLUENCE  OF  ENVIRONMENT  313 

offspring,  though  this  is  by  no  means  proved; 
while  Hertwig  concludes  that  the  great  pre- 
valence of  the  drug  habit  may  seriously  affect 
the  germ  cells  and  their  subsequent  develop- 
ment. Forel  has  for  many  years  maintained 
that  one  of  the  most  serious  causes  of  human 
malformations  and  degenerations  is  to  be 
found  in  the  effect  of  alcohol  on  the  germ  cells, 
especially  at  the  time  of  conception. 

2.  Fertilization  Stages.  —  Environmental 
changes  acting  during  fertilization  may  cause 
more  than  one  spermatozoon  to  enter  the  egg 
or  may  injure  the  egg  or  sperm;  in  either 
case  the  resulting  development  is  abnormal. 
Where  two  or  more  spermatozoa  enter  the 
egg  the  nuclear  divisions  are  usually  abnormal, 
as  Boveri  has  shown  in  the  case  of  the  sea 
urchin;  the  distribution  of  chromosomes  to 
different  cleavage  cells  is  unequal  and  such 
cells  do  not  undergo  typical  development, 
while  the  embryo  or  larva  produced  is  not 
capable  of  continued  life.  In  cases  where 
an  egg  is  fertilized  by  a  spermatozoon  belong- 
ing to  a  different  phylum  or  class  (heterogen- 
eous fertilization)  the  foreign  sperm,  after 


314  HEREDITY  AND  ENVIRONMENT 

stimulating  the  egg  to  begin  development,  may 
itself  die  or  remain  inactive,  in  which  case  the 
hereditary  traits  which  develop  are  those  of  the 
mother  only.  In  many  animals  unfertilized 
eggs  may  be  stimulated  to  begin  development 
by  a  great  variety  of  changes  in  the  medium, 
all  such  cases  being  known  as  "artificial 
parthenogenesis . ' ' 

3.  Modifications  of  Development  after 
Fertilization. — Environmental  changes,  acting 
upon  the  oosperm  after  fertilization,  or  upon 
the  embryo,  may  produce  an  almost  infinite 
variety  of  abnormal  types  of  development,  but 
so  far  as  known  none  of  these  modifications  be- 
comes hereditary.  Changes  in  hereditary  con- 
stitution take  place  in  the  main  before  fertili- 
zation and  especially  during  the  maturation 
divisions. 

If  the  cleavage  cells  are  separated  from  one 
another  in  the  2-cell  or  4-cell  stage  each  of 
them  may  give  rise  to  an  entire  animal  (Fig. 
70)  ;  in  this  way  four  complete  animals  may 
be  derived  from  a  single  egg  of  a  star-fish  or 
sea  urchin,  of  an  amphioxus,  or  of  several 
other  animal  types.  If  the  frog's  egg  is  turned 


315 


INFLUENCE  OF  ENVIRONMENT 

upside  down  in  the  2-cell  stage,  double  headed 
or  double  bodied  embryos  may  result  (Fig. 
71 ).  In  such  cases  each  cleavage  cell  is  said  to 


FIG.  70.  Dwarf  and  double  embryos  of  AmpTiioxus.  A, 
isolated  blastomeres  of  the  2-cell  stage  segmenting  like  an 
entire  egg.  B,  twin  gastrulae  from  a  single  egg.  C,  double 
cleavage  resulting  from  partial  separation  of  the  first  two 
cleavage  cells.  D,  E,  F,  double  gastrulae  arising  from  such 
forms  as  the  last.  (From  Wilson.) 


316 


HEREDITY  AND  ENVIRONMENT 


be  totipotent,  that  is,  it  is  capable  of  giving 
rise  to  an  entire  animal. 

On  the  other  hand  in  certain  animal  phyla 
such  as  the  ctenophores,  mollusks,  annelids 
and  ascidians  isolated  cleavage  cells  give  rise 


D 


FIG.  71.  Double  embryos  of  frog  developed  from  eggs  in- 
verted when  in  the  2-cell  stage.  A,  twins  with  heads  turned 
in  opposite  directions.  B,  twins  united  back  to  back.  C, 
twins  united  by  their  ventral  sides.  D,  double  headed  tad- 
pole. (From  Wilson  .after  O.  Schultze.) 


INFLUENCE  OF  ENVIRONMENT 


317 


m'ch. 


E 

FIG.  72.     Half    and    three-quarter    embryos    of    Styela,  np 

nerve    plate,    nt    nerve    tube,    E    eye,    mch    mesenchyme,  ma 

muscle,     ch     notochord.     A,     right     half-gastrula     which  de- 


318  HEREDITY  AND  ENVIRONMENT 

only  to  parts  of  an  animal ;  in  this  way  one  may 
get  a  right  or  left  half  of  an  animal  (Fig.  72) 
from  right  or  left  cleavage  cells;  an  anterior 
half  (Fig.  73),  or  a  posterior  half  (Fig.  74) 
from  anterior  or  posterior  cleavage  cells;  or 
any  one  of  the  cells  of  the  4-cell  stage  may 
produce  the  corresponding  quarter  of  an  entire 
animal.  Such  cases  are  known  as  "mosaic 
development." 

There  has  been  much  discussion  among  bi- 
ologists as  to  the  meaning  of  these  results.  On 
the  one  hand  it  is  said  that  the  totipotence 
of  any  one  of  the  first  four  cleavage  cells 
proves  that  all  of  these  cells  are  alike  and  that 
they  have  not  yet  begun  to  differentiate.  On 
the  other  hand  it  is  said  that  a  part  of  an  egg 
may  give  rise  to  a  whole  animal  for  the  same 
reason  that  parts  of  certain  adult  animals  may 
do  the  same  thing,  viz.,  because  they  have  the 
power  of  regeneration.  However  there  are 


veloped  after  the  left  half  of  the  egg,  A3BX,  had  been  killed. 
B,  left  half  larva  from  the  two  left  cells  of  the  4-cell  stage, 
the  right  cells,  <4SBS,  having  been  killed.  The  muscle  cells 
(stippled)  occur  only  on  one  side  of  the  notochord.  D, 
three-quarter  larva,  the  left  anterior  cells  having  been  killed. 
E,  F,  three-quarter  larvae,  the  right  posterior  cell  B,  having 
been  killed. 


FIG.  73.     Anterior  half-embryos  of  Styela,  the   posterior  cells 
having  been  killed  by  the  4-cell  stage. 


FIG.  74.     Posterior  half-embryos  of  Styela,  the  anterior  cells 
having  been  killed  in  the  4-cell  stage. 


INFLUENCE  OF  ENVIRONMENT  321 

many  animals  which  are  incapable  of  regener- 
ating lost  parts  of  their  bodies,  and  similarly 
there  are  cases  in  which  part  of  an  egg  cannot 
give  rise  to  a  whole  animal.  The  evidence 
available  at  present  favors  the  view  that  in 
cases  where  one  of  the  cleavage  cells  is  capable 
of  giving  rise  to  a  whole  animal  there  is  a 
greater  capacity  of  regeneration  or  regula- 
tion, and  possibly  also  a  lower  degree  of  initial 
differentiation,  than  in  those  cases  in  which 
part  of  an  egg  is  capable  of  producing  only 
part  of  an  animal. 

If  the  fertilized  egg  is  whirled  rapidly  on  a 
centrifugal  machine  it  may  be  subjected  to  a 
pressure  several  hundred  times  that  of  gravity. 
Under  such  conditions  the  heavier  particles 
are  thrown  to  one  side  of  the  egg  and  the 
entire  substance  of  the  egg  becomes  stratified 
into  layers  or  zones.  In  the  ascidian  egg, 
where  the  different  kinds  of  protoplasm  give 
rise  to  different  tissues  and  organs,  this  rear- 
rangement of  the  egg  substances  may  lead  to  a 
marked  dislocation  of  organs ;  the  animal  may 
be  turned  inside  out,  having  the  endoderm  on 
the  outside  and  its  skin  and  ectoderm  on  the 
inside,  etc.  (Fig.  75). 


322 


HEREDITY  AND  ENVIRONMENT 


If  the  cleavage  cells  are  only  partially  sep- 
arated they  may  produce  animals  which  are 
partially  separated,  such  as  Siamese  twins, 
two-headed  forms,  etc.  (Fig.  71).  Or  these 
double  monsters  may  be  produced  by  division 
or  budding  of  the  embryo  at  a  later  stage  of 
development.  In  the  human  species,  no  less 
than  in  other  animals,  all  sorts  of  double  mon- 
sters may  be  formed  in  this  way  by  the  partial 
division  of  a  single  egg  or  embryo  (Fig.  76). 
If  the  division  is  slight  the  developed  indi- 
vidual may  show  only  the  beginnings  of  a  divi- 


FIG.  75.  Two  larvae  of  Styela  which  were  centrifuged  in 
the  4-celled  stage  thereby  changing  the  position  of  various 
organ-forming  substances.  Nervous  system  («#),  eyes  (£?), 
notochord  (ch)  and  muscles  (ms)  have  been  displaced,  and 
the  larva  has  been  turned  inside  out,  the  endoderm  (end) 
being  outside  and  the  ectoderm  (ect)  inside. 


INFLUENCE  OF  ENVIRONMENT  323 

sion  into  two,  as  in  two-headed  forms;  if  the 
division  of  the  egg  or  embryo  is  complete  two 
separate  and  perfect  individuals  may  be 
formed  from  an  original  single  oosperm. 
When  two  individuals  are  formed  from  a 
single  egg  they  have  exactly  the  same  he- 
redity and  accordingly  they  are  always  of  the 
same  sex  and  are  so  similar  in  appearance  that 
they  are  known  as  "identical"  or  "duplicate" 
twins  (Fig.  76).  On  the  other  hand  twins 
which  develop  from  different  eggs  do  not  have 


FIG.  76.  Diagram  showing  the  different  types  of  union  of 
double  human  monsters  each  being  produced  by  a  partial 
division  of  a  single  egg  or  embryo.  If  the  division  is  a 
complete  one,  duplicate  twins  are  formed,  as  shown  on  the 
right.  (From  Wilder.) 


324  HEREDITY  AND  ENVIRONMENT 

the  same  heredity  and  may  differ  in  sex  as  well 
as  in  other  features;  they  are  known  as  "fra- 
ternal" twins. 

If  the  temperature  or  density  of  the  sur- 
rounding medium  is  altered  during  the  gas- 
trula  stages  the  endoderm  may  be  caused  to 
turn  out  instead  of  in  (exogastrula),  thus  pro- 
ducing an  animal  which  is  turned  inside  out 
(Fig.  77).  In  other  cases  (vertebrates)  the 
gastrula  mouth  may  fail  to  close,  thus  pro- 
ducing animals  in  which  the  spinal  cord  and 
vertebral  column  are  split  in  two  (spina 


FIG.  77.  Exogastrula  of  Crepidula.  The  endoderm  (end) 
has  been  turned  out  instead  of  in,  thus  leaving  the  digestive 
layer  of  cells  on  the  outside  of  the  body;  ShG,  shell  gland, 
V,  velum. 


INFLUENCE  OF  ENVIRONMENT 


325 


bifida)  ;  or  the  brain  may  be  forced  outside  of 
the  head  or  may  be  lacking  altogether  (anen- 
cephaly)  .  In  some  cases  eyes  are  wholly  lack- 
ing, in  others  the  two  eyes  fuse  together  into  a 
single  one  as  in  the  fabled  Cyclops  (Fig.  78). 
Practically  all  such  cases  of  monstrous  devel- 
opment are  due  to  abnormal  environmental 
conditions  in  early  stages  of  development. 

In  addition  to  such  monsters,  which  are  in- 
capable of  long  life,  many  peculiar  if  not  ab- 
normal types  of  animals  are  produced  by  the 


FIG.  78.  Young  fish;  on  the  right  a  normal  individual  with 
two  eyes;  on  the  left  cyclopean  monsters  with  one  eye; 
produced  by  treatment  with  magnesium  solutions.  (From 
Stockard.) 


326 


HEREDITY  AND  ENVIRONMENT 


action  of  unusual  environmental  stimuli  dur- 
ing later  stages  of  development.  Gudernatsch 
found  that  if  tadpoles  of  the  frog  were  fed  on 
the  thyroid  gland  they  transformed  into 
minute  frogs,  scarcely  larger  than  flies,  but  if 
fed  on  thymus  gland  they  grew  to  be  large, 
dark  colored  tadpoles  but  did  not  change  into 
frogs;  if  fed  on  the  adrenal  gland  they  pro- 
duced extremely  light  colored  forms.  If  canary 
birds  are  fed  on  sweet  red  pepper  they  become 
red  in  color.  If  the  larvae  of  bees  are  fed.  on 
"royal  jelly,"  which  is  a  bee  food  rich  in  fats, 
they  become  fertile  females  or  queens;  if  fed 
on  ordinary  "bee  bread"  they  become  infertile 
females  or  workers  (Fig.  79).  There  are 


FIG.  79.  The  three  castes  of  the  honey  bee.  A,  worker  or 
imperfect  female,  B,  queen  or  perfect  female,  C,  drone  or 
male.  The  differences  between  workers  and  queens  are  pro- 
duced by  the  type  of  food  supplied  to  the  larvae. 


INFLUENCE  OF  ENVIRONMENT  327 

marked  structural  differences  between  the 
workers  and  the  queens  but  the  differences  in 
their  habits  and  instincts  are  even  more  strik- 
ing; all  of  these  differences  whether  in  bodily 
structure  or  in  instincts  are  determined  by  the 
character  of  the  food  and  not  by  heredity. 
Innumerable  cases  of  a  similar  sort  could  be 
named  which  show  the  great  effect  of  environ- 
mental stimuli  on  development. 

C.  FUNCTIONAL   ACTIVITY   AS    A   FACTOR 
OF  DEVELOPMENT 

Another  factor  of  development  which  is 
partly  intrinsic  and  partly  extrinsic  is  func- 
tional activity  or  use.  Functional  activity  is 
response  to  stimuli  which  may  be  external  or 
internal  in  origin.  The  entire  process  of  de- 
velopment may  be  regarded  as  a  series  of  such 
responses  on  the  part  of  the  organism,  whether 
germ  cell,  embryo  or  adult,  to  such  stimuli. 
The  nature  of  the  response  is  determined  by 
the  nature  and  state  of  the  organism  at  the 
time  and  by  the  character  of  the  stimulus.  By 
the  normal  or  usual  series  of  stimuli  certain 


328  HEREDITY  AND  ENVIRONMENT 

parts  are  kept  active  while  other  parts  are 
kept  inactive  or  are  inhibited. 

Normal  development  is  dependent  upon  the 
correlated  activity  of  many  parts  of  the  or- 
ganism. If  in  any  part  stimuli  and  responses 
are  lacking  the  development  of  that  part  is 
arrested  or  inhibited.  For  example  in  the 
cleavage  stages  different  kinds  of  plasm  are 
sorted  and  localized  by  protoplasmic  move- 
ments within  cells;  these  substances  are  iso- 
lated by  cell  divisions  and  by  the  formation  of 
partition  walls  between  cells,  which  are 
brought  about  by  protoplasmic  movements  in 
response  to  stimuli;  if  these  movements  are 
stopped  cleavage  and  differentiation  are  ar- 
rested. In  later  stages  the  infolding  of  the 
gastrula,  the  neural  tube,  the  alimentary  canal, 
and  the  folding  of  layers  in  general,  which 
plays  so  important  a  part  in  development,  are 
due  to  the  movements  of  substances  within 
cells  and  to  the  movements  of  cells  in  the 
layers  in  which  they  lie,  and  if  these  move- 
ments be  inhibited  normal  development  is 
prevented. 

Another  type  of  functional  activity  which  is 


INFLUENCE  OF  ENVIRONMENT  329 

a  potent  factor  in  development  is  found  in  the 
trophic  or  nutritive  relations  which  exist  be- 
tween different  parts  of  the  organism.  Or- 
gans long  unused  undergo  regressive  changes 
and  may  become  rudimentary,  for  example 
the  muscles  of  a  limb  which  has  been  par- 
alyzed or  placed  in  a  cast  shrivel ;  on  the  other 
hand  use  increases  the  size  and  strength  of  any 
organ.  Inactivity  or  atrophy  of  one  part  usu- 
ally leads  to  imperfect  nourishment  and  de- 
velopment of  related  parts;  for  example,  the 
optic  nerve  atrophies  when  the  eye  is  lost,  and 
muscles  atrophy  when  the  nerves  leading  to 
them  are  destroyed  or  paralyzed.  In  general 
the  normal  development  of  any  part  is  de- 
pendent upon  its  proper  nutrition  and  this  is 
dependent  upon  the  functional  activity  of  this 
and  of  other  related  parts. 

Still  another  phase  of  functional  activity  is 
found  in  the  effects  of  certain  secretions  and 
chemical  substances  which  are  formed  by  dif- 
ferent glands.  In  many  cases  the  secondary 
sexual  characters  which  distinguish  the  male 
and  the  female  are  due  to  chemical  substances 
from  the  ovary  or  the  testes,  which  stimulate  or 


330  HEREDITY  AND  ENVIRONMENT 

inhibit  the  formation  of  these  characters.  If 
the  ovary  is  removed  from  a  young  hen  she  de- 
velops the  larger  size,  the  more  brilliant  plum- 
age and  the  peculiar  comb,  wattles  and  spurs 
of  the  cock.  These  secondary  sexual  charac- 
ters of  the  male  are  potential  in  the  female 
but  are  kept  from  developing  or  are  inhibited 
by  the  activity  of  the  ovary.  On  the  other 
hand  the  castration  of  the  young  cock  does  not 
prevent  the  development  of  most  of  the  sec- 
ondary sexual  characters  of  the  male.  In  the 
case  of  mammals  removal  of  the  ovaries  of  a 
young  female  or  of  the  testes  of  a  young  male 
does  not  lead  to  the  development  of  the  sec- 
ondary sexual  characters  of  the  other  sex,  but 
both  sexes  remain  in  a  sexually  undeveloped 
or  infantile  condition,  that  is,  the  presence  of 
ovaries  or  testes  serves  as  stimulus  to  call  forth 
the  development  of  the  secondary  sexual  char- 
acters in  mammals,  and  not  as  inhibitors  to 
prevent  the  development  of  the  secondary 
sexual  characters  of  the  opposite  sex,  as  in  the 
female  fowl.  If  bits  of  the  ovary  of  a  guinea- 
pig  are  inserted  under  the  skin  of  a  young 
male  which  has  been  previously  castrated,  the 


INFLUENCE  OF   ENVIRONMENT  331 

latter  develops  mammary  glands  similar  to 
those  of  a  normal  female;  in  short  he  is  "femin- 
ized" by  the  stimulus  of  substances  from  the 
ovary. 

Another  gland  whose  secretions  exercise  a 
profound  influence  on  development  is  the 
thyroid,  which  is  found  in  the  neck  near  the 
"Adam's  apple."  If  this  gland  becomes  en- 
larged it  gives  rise  to  goitre,  protruding  eye- 
balls, rapid  heart  beat ;  on  the  other  hand  if  the 
thyroid  is  deficient  in  a  young  child  it  causes  a 
peculiar  type  of  idiotic  dwarf  known  as  "cretin" 
( Fig.  68 ).  If  the  gland  which  lies  between  the 
roof  of  the  mouth  and  the  base  of  the  brain 
and  which  is  known  as  the  hypophysis  is  de- 
ficient the  child,  or  young  animal,  remains  in- 
fantile ;  if  the  hypophysis  is  too  large  the  indi- 
vidual's hands,  feet  and  face  become  enlarged 
and  he  may  grow  to  be  a  deformed  giant,  but 
with  weak  body  and  mind. 

Many  cases  are  known  in  which  the  develop- 
ment of  a  part  is  dependent  upon  the  presence 
of  another  part;  this  is  technically  known  as 
"correlative  differentiation."  Thus  it  has  been 
found  that  the  lens  of  the  eye  will  develop 


332  HEREDITY  AND  ENVIRONMENT 

from  any  portion  of  the  ectoderm,  or  outer 
layer  of  the  skin,  if  only  the  primitive  retina,  or 
optic  cup,  is  brought  near  to  this  layer;  if  the 
optic  cup  is  transplanted  from  the  head  to  the 
thorax  or  abdomen  a  lens  will  form  wherever 
the  cup  comes  in  contact  with  the  ectoderm. 
If  an  embryonic  limb  is  transplanted  from  its 
normal  position  to  the  middle  of  the  back  or 
belly,  it  will  develop,  and  nerves  and  blood  ves- 
sels will  grow  into  it  which  would  have  had 
very  different  positions  and  distributions  if 
the  limb  had  not  been  there.  If  one  of  the 
first  four  cleavage  cells  is  separated  from 
the  others  it  may  develop  into  an  entire  animal 
though  it  would  have  formed  only  a  quarter 
of  an  animal  if  it  had  remained  in  contact  with 
the  other  three-quarters  of  the  egg.  All  such 
cases  are  known  as  "correlative  differentia- 
tion," implying  that  differentiation  is  depend- 
ent upon  the  stimuli  which  come  from  sur- 
rounding parts.  On  the  other  hand  if  the 
differentiation  has  already  begun  before  the 
relation  of  a  part  to  surrounding  parts  has 
been  changed,  it  may  continue  to  differentiate 
as  if  no  change  of  position  or  relation  had 


INFLUENCE  OF  ENVIRONMENT  333 

taken  place.  Thus  if  a  right  limb  is  trans- 
planted to  the  left  side  of  the  body  after  it 
has  begun  to  differentiate  it  remains  a  right 
limb  and  is  not  modified  by  its  new  relations 
(Harrison)  ;  if  the  cleavage  cells  Are  already 
differentiated  in  the  four-celled  stage,  each 
cell  when  separated  from  the  others  will  give 
rise  to  only  one-quarter  of  an  animal.  In  short 
the  organ  or  cell  is  already  set,  or  fixed,  or 
differentiated  to  such  an  extent  that  it  can  not 
change  its  fate  even  though  its  environment 
should  change.  Such  cases  are  known  as  "self 
differentiation." 

Many  students  of  the  physiology  of  devel- 
opment have  been  led  to  the  view  that  the  fun- 
damental causes  of  development  are  to  be 
found  not  in  the  egg  cell  itself  but  in  en- 
vironmental stimuli  and  in  the  interaction  of 
the  various  parts.  Driesch  in  particular  re- 
gards the  egg,  or  any  cleavage  cell,  as  a  "har- 
monic equipotential  system,"  that  is,  any  part 
is  capable  of  any  fate,  and  its  actual  fate  is 
determined  by  its  relation  to  other  parts;  in 
the  striking  phrase  of  Driesch,  "The  fate  of 
a  part  is  a  function  of  its  position."  We  now 


334  HEREDITY  AND  ENVIRONMENT 

know  that  this  expresses  only  a  fraction  of  the 
truth.  The  fate  of  a  part  is  primarily  a  func- 
tion of  its  organization  and  only  secondarily  a 
function  of  its  position. 

These  are  only  a  few  illustrations  of  the 
many  kinds  of  abnormal  development  which 
may  be  caused  by  changed  environment  or  by 
unusual  functional  activities.  At  all  stages  of 
ontogeny  the  course  of  development  may  be 
altered  by  extrinsic  stimuli  but  earlier  stages 
may  be  more  profoundly  influenced  than  later 
ones. 


D.  INHERITANCE    OR    NON-INHERITANCE 
OF  ACQUIRED  CHARACTERS 

Few  questions  in  biology  have  been  dis- 
cussed so  fully  and  so  fruitlessly  as  this.  It  is 
a  problem  of  the  greatest  interest  not  only  to 
students  of  biology  but  also  to  sociologists, 
educators  and  philanthropists  and  yet  it  is  still 
to  a  great  extent  an  unsolved  problem.  It  is 
well  known  that  Lamarck  and  his  followers 
taught  that  characters  due  to  the  conditions  of 
life,  or  to  use  and  disuse,  were  inherited.  Dar- 


INFLUENCE  OF  ENVIRONMENT  335 

win  also  held  this  view  and  proposed  his  hy- 
pothesis of  pangenesis  in  order  to  explain  the 
process  of  the  transmission  of  such  characters 
to  the  germ  cells. 

Weismann  introduced  a  new  era  in  biology 
by  denying  the  inheritance  of  acquired  char- 
acters, by  showing  the  weakness  of  the  evi- 
dence for  such  inheritance  and  by  challenging 
the  world  to  produce  evidence  that  would  stand 
a  rigorous  analysis.  But  Weismann's  great- 
est service  lay  in  his  constructive  theories 
rather  than  in  destructive  criticism ;  he  forever 
disposed  of  theories  of  pangenesis  and  the  like 
by  showing  that  the  germ  cells  are  not  built 
up  by  contributions  from  the  body  and  that 
characters  are  not  transmitted  from  genera- 
tion to  generation ;  but  on  the  other  hand  that 
there  is  transmitted  a  germ  plasm  which  is  rela- 
tively independent  of  the  body  and  which  is 
relatively  very  stable  in  organization.  This 
epoch-making  theory  of  Weismann's  has 
naturally  undergone  some  changes,  as  the  re- 
sult of  new  discoveries.  It  is  no  longer  be- 
lieved that  the  germ  plasm  is  really  independ- 
ent of  the  body,  nor  that  it  is  absolutely  stable, 


336  HEREDITY  AND  ENVIRONMENT 

as  Weismann  at  one  time  held.  There  is  no 
doubt  that  the  germ  cells  and  the  germ  plasm 
are  physiologically  related  to  other  cells  and 
to  other  plasms,  and  similarly  there  is  no  doubt 
that  the  germ  plasm  although  very  stable  can 
and  does  change  its  constitution  under  some, 
rare  conditions.  But  in  the  main  r  the  germ 
plasm  theory  is  accepted  by  the  great  majority 
of  biologists  to-day,  and  recent  work  in  gen- 
etics and  cytology  has  brought  many  confir- 
mations of  this  theory. 

As  long  as  it  was  believed  that  the  developed 
characters  of  an  organism  could  be  transmit- 
ted as  such  to  its  descendents  it  was  customary 
to  speak  of  developed  characters  as  heredi- 
tary or  acquired  and  to  talk  of  the  inheritance 
or  non-inheritance  of  acquired  characters. 
This  distinction  is  not  a  logical  one  for  all 
developed  characters  are  invariably  the  result 
of  the  responses  of  the  germinal  organi- 
zation to  environmental  stimuli ;  and  of  course 
no  developed  character  can  be  purely  heredi- 
tary or  purely  environmental.  But  when  a 
given  character  arises  in  many  individuals  of 
the  same  genotype  under  different  environ- 


INFLUENCE  OF   ENVIRONMENT  337 

mental  conditions  it  is  probable  that  heredity, 
which  is  the  constant  factor  in  this  case,  is  also 
the  determining  factor  for  that  character.  On 
the  other  hand  if  a  character  develops  in  re- 
sponse to  peculiar  stimuli  and  does  not  appear 
in  other  individuals  of  the  same  genotype  in 
which  such  stimuli  are  lacking  it  is  said  to  be 
an  environmental  or  acquired  character. 

But  though  environment  and  functional  ac- 
tivity exercise  a  great  influence  on  the  course 
of  development  there  is  no  sufficient  evidence 
that  they  produce  corresponding  changes  in 
heredity.  Hereditary  constitution  is  not  gen- 
erally changed  by  accidents  of  environment, 
but  individual  development  is  so  changed. 

Briefly  stated  the  question  of  the  inheritance 
of  acquired  characters  is  this:  Can  peculiari- 
ties of  the  environment  which  bring  about  the 
development  of  somatic  characters  so  affect  the 
germ  cells  that  they  will  produce  these  so- 
matic characters  in  the  absence  of  the  peculiar 
environment?  Can  the  characteristics  of  a  de- 
veloped organism  enter  into  its  germ  cells  and 
be  born  again  in  the  next  generation?  Con- 
sidering the  fact  that  germ  cells  are  cells  and 


338  HEREDITY  AND  ENVIRONMENT 

contain  no  adult  characteristics,  it  seems  very 
improbable  that  any  peculiarity  of  environ- 
ment whether  of  nutrition,  use,  disuse  or  in- 
jury, which  brings  about  certain  peculiarities 
of  developed  characters  in  the  adult,  could  so 
change  the  structure  of  the  germ  cells  as  to 
cause  them  to  produce  this  same  character  in 
subsequent  generations  in  the  absence  of  its 
extrinsic  cause.  How,  for  example,  could  de- 
fective nutrition,  which  leads  to  the  produc- 
tion of  rickets,  affect  the  germ  cells,  which 
contain  no  bones,  so  as  to  produce  rickets  in 
subsequent  generations,  although  well  nour- 
ished? Or,  how  could  overexertion,  leading  to 
hypertrophy  of  the  heart,  so  affect  the  future 
germ  cells  that  they,  in  turn,  would  produce 
hypertrophied  hearts  in  the  absence  of  over- 
exertion,  seeing  that  germ  cells  have  no  hearts  ? 
Or  how  could  the  loss  or  injury  of  eyes  or 
teeth  or  legs  lead  to  the  absence  or  weakened 
development  of  these  organs  in  future  genera- 
tions, seeing  that  inheritance  must  be  through 
germ  cells  which  possess  none  of  these 
structures? 

But,  apart  from  these  general  objections  to 


INFLUENCE  OF  ENVIRONMENT  339 

the  doctrine  of  the  inheritance  of  acquired 
characters,  there  are  many  special  difficulties. 
There  is  no  conclusive  and  satisfactory  evi- 
dence in  favor  of  such  inheritance.  Almost  all 
the  evidence  adduced  serves  to  show  only  that 
characters  are  acquired,  not  that  these  charac- 
ters are  inherited. 

It  is  a  matter  of  common  observation  that 
mutilations  are  not  inherited;  wooden  legs  do 
not  run  in  families,  although  wooden  heads  do. 
The  evidence  for  the  inheritance  of  peculiari- 
ties due  to  excessive  use  or  disuse  is  wholly  in- 
conclusive, and  in  general  the  same  may  be 
said  of  any  special  character  due  to  abnormal 
nutrition.  That  unusual  conditions  of  food, 
temperature,  moisture,  etc.,  may  affect  the 
germs  of  future  generations  so  as  to  produce 
general  and  indefinite  variations  is  very  prob- 
able, but  this  is  a  very  different  thing  from  the 
inheritance  of  acquired  characters.  The  germ 
cells  being  a  part  of  the  parental  organism 
may  be  modified  by  such  changes  in  the  en- 
vironment as  affect  the  body  as  a  whole,  they 
may  be  well  nourished  or  starved,  they  may  be 
modified  by  changed  conditions  of  gravity, 


340  HEREDITY  AND  ENVIRONMENT 

salinity,  pressure,  temperature,  etc.,  and  these 
modifications  of  the  germ  cells  may  and  prob- 
ably do  lead  to  certain  general  modifications 
of  the  adult,  which  may  be  larger  or  smaller, 
stronger  or  weaker,  according  as  the  germ  is 
well  or  poorly  nourished,  etc.  It  is  further 
possible  that  these  changed  environmental 
conditions  may  bring  about  such  changes  in 
the  structure  of  the  germ  cells  as  to  produce 
great  and  remarkable  peculiarities  in  the 
adult ;  but  it  is  inconceivable  that  the  environ- 
ment which  produces  rickets,  or  hypertrophied 
heart,  or  loss  of  sight  in  one  generation,  should 
modify  the  germ  cells  of  the  next  generation 
in  such  a  peculiar  and  definite  way  that  they 
should  give  rise  to  these  particular  peculiari- 
ties, in  the  absence  of  the  extrinsic  cause  which 
first  produced  them.  The  inheritance  of  ac- 
quired characters  is  inconceivable,  because 
the  egg  is  a  cell  and  not  an  adult  organism; 
and  in  this  case  there  is  no  sufficient  evidence 
that  the  thing  which  is  inconceivable  really 
does  happen. 

If  environmental  conditions  may  alter  he- 
reditary constitution  we  should  expect  to  find 


INFLUENCE  OF  ENVIRONMENT  341 

that  where  plants  or  animals  are  grafted  to- 
gether each  would  modify  more  or  less  the 
hereditary  constitution  of  the  other.  But  this 
does  not  occur.  Everybody  knows  that  when 
a  branch  of  a  particular  kind  of  fruit  tree  is 
grafted  upon  a  tree  of  a  different  variety  the 
quality  of  the  fruit  borne  by  that  branch  is 
not  altered  by  its  close  union  with  the  new 
stock.  The  same  is  true  of  all  forms  of  ani- 
mal grafts.  Harrison  cut  in  two  young  tad- 
poles of  two  species  of  frog,  Rana  sylvatica 
and  Rana  palustris,  and  spliced  the  anterior 
half  of  one  to  the  posterior  half  of  the  other. 
These  frogs  and  their  tadpoles  differ  in  color 
as  well  as  in  other  respects,  R.  sylvatica  being 
more  deeply  pigmented  than  R.  palustris.  In 
the  grafted  tadpoles  each  half  preserved  its 
own  peculiarities  even  up  to  the  adult  condi- 
tion (Fig.  80). 

A  still  more  striking  case  of  the  persistence 
of  heredity  in  spite  of  environmental  changes 
is  found  in  experiments  in  which  the  ovaries 
are  removed  from  one  variety  of  animal  and 
transplanted  to  another  variety.  Guthrie 
made  such  transplantations  in  the  case  of 


342 


HEREDITY  AND  ENVIRONMENT 


fowls  and  concluded  that  there  was  some  in- 
fluence of  the  foster  mother  upon  the  trans- 
planted ovary,  but  Davenport,  who  repeated 
his  experiments,  was  unable  to  confirm  his  re- 
sults. Finally  Castle  and  Phillips  furnished 
the  most  conclusive  demonstration  that  the 


FIG.  80.  Grafted  frog  embryos,  anterior  part,  Rana  syl- 
vatica,  posterior  part,  R.  palustris.  In  later  stages,  and  even 
in  the  adult  condition,  the  two  parts  preserve  their  peculiari- 
ties. (From  Harrison.) 


INFLUENCE  OF   ENVIRONMENT  343 

hereditary  characteristics  of  the  transplanted 
ova  are  in  no  wise  changed  by  .  the  foster 
mother.  They  removed  the  ovary  from  a  pure 
•black  guinea-pig  and  put  it  in  the  place 
of  the  ovary  of  a  pure  white  animal.  After 
recovery  from  the  operation  this  white  female 
with  the  "black"  ovary  was  bred  to  a  pure 
white  male  (Fig.  81).  Three  litters  of  off- 
spring from  these  parents  were  all  pure  black 
as  shown  in  Figure  82.  Although  both  parents 
were  pure  white  all  the  offspring  of  the  FI 
generation  were  black  because  they  came  from 
"black"  eggs  and  black  is  dominant  over  white. 
The  fact  that  these  "black"  eggs  matured  in 
the  body  of  a  white  female  did  not  in  the  least 
change  their  hereditary  constitution. 

A  still  more  intimate  union  takes  place  when 
the  dominant  and  recessive  characters  come 
together  in  any  zygote.  These  characters,  or 
rather  the  factors  which  determine  them,  may 
be  intimately  associated  in  every  cell  of  the 
organism  throughout  an  entire  generation  and 
yet  we  may  get  a  clean  separation  of  these 
characters  in  the  next  generation;  in  many 
cases  neither  the  dominant  nor  the  recessive 


344  HEREDITY  AND  ENVIRONMENT 


FIG.  81.     Effect    of    transplanting    ovaries    in    guinea-pigs. 


INFLUENCE  OF  ENVIRONMENT  345 

character  has  been  at  all  modified  by  its  most 
intimate  association  with  the  other. 

A  striking  instance  of  the  purely  temporary 
effect  of  the  environment  and  of  the  long  per- 
sistence of  hereditary  constitution  amidst  new 
environmental  conditions,  which  have  greatly 
changed  the  appearance  of  the  developed  or- 
ganism, is  found  in  the  case  of  alpine  plants. 
Xageli  says  that  such  plants,  which  have  pre- 
served the  characters  of  high  mountain  plants 
since  the  ice  age,  lose  these  characters  perfectly 
during  their  first  summer  in  the  lowlands. 

If  acquired  characters  were  really  inherited 
we  should  expect  to  find  many  positive  evi- 
dences of  this  instead  of  a  few  sporadic  and 
doubtful  cases.  In  particular  why  do  we  not 
find  in  plant  or  animal  grafting  that  the  in- 
fluence of  the  stock  changes  the  hereditary  po- 
tencies of  the  graft?  Why  do  we  not  find 
that  transplanted  ovaries  show  the  influence  of 


Above,  young  black  female;  in  the  middle,  mature  white  fe- 
male; below,  mature  white  male.  The  white  female's  ovary 
was  removed  and  in  its  place  was  put  the  ovary  from  the 
black  female.  The  white  female  (with  "black"  ovary)  was 
then  bred  to  the  white  male.  (From  Castle.) 


346  HEREDITY  AND  ENVIRONMENT 


FIG.  82.     Results  of  cross  described  in  the  preceding  figure. 


INFLUENCE  OF  ENVIRONMENT  347 

the  foster  mother  as  Guthrie  supposed — a 
thing  which  has  been  disproved  by  Castle 
(Figs.  81  and  82)?  Why  do  dominant  and 
recessive  characters  remain  pure,  even  after 
their  intimate  union  in  a  hybrid,  so  that  pure 
dominants  and  pure  recessives  may  be  obtained 
in  subsequent  generations  from  this  mixture? 
Why  does  every  child  have  to  learn  anew  what 
his  parents  learned  so  laboriously  before  him? 
Even  the  strongest  defenders  of  the  inheri- 
tance of  acquired  characters  are  constrained 
to  admit  that  it  occurs  only  sporadically  and 
exceptionally. 

Many  modifications  of  the  Lamarckian  hy- 
pothesis of  the  inheritance  of  acquired  char- 
acters have  been  proposed  in  recent  years. 
Foremost  among  these  are  the  "mneme" 
theory  of  Semon  and  the  "centro-epigenesis" 
theory  of  Rignano.  To  Semon  as  to  many 
other  biologists  the  apparent  resemblance  be- 
tween memory  and  heredity  has  seemed  sig- 


All  the  offspring  are  pure  black,  though  both  parents  are 
pure  white,  because  the  white  female  contains  only  "black" 
eggs  and  black  is  dominant  over  white.  (From  Castle.) 


348  HEREDITY  AND  ENVIRONMENT 

nificant,  and  this  furnishes  the  basis  of  his 
theory.  Semon  holds  that  every  condition  of 
life,  every  functional  activity  of  an  organism 
leaves  a  permanent  record  of  itself  in  what  he 
calls  an  "engramme."  If  these  conditions  or 
activities  are  long  continued  their  engrammes 
are  heaped  up  and  affect  heredity.  Se- 
mon does  riot  ask  if  "acquired  characters"  are 
inherited,  but  rather  "Are  the  hereditary  po- 
tencies of  the  germ  cells  altered  by  stimuli  act- 
ing on  the  parental  body?"  This  is  a  very 
different  thing  from  the  inheritance  of  a  par- 
ticular acquired  character,  and  there  is  evi- 
dence that  such  stimuli  may  in  some  instances 
produce  changes  in  the  hereditary  constitution 
of  the  germ  cells,  especially  at  or  near  their 
maturation  stages.  The  observations  and  ex- 
periments of  deVries  on  the  evening  primrose, 
of  Hansen  on  yeast  and  of  Fischer,  Standfuss. 
Tower  and  Morgan  on  the  production  of  mu- 
tations in  insects  favor  such  a  belief. 

On  the  other  hand  certain  changes  may  be 
produced  in  germ  cells  or  embryos  which  last 
only  for  a  generation  or  two  and  then  disap- 
pear. It  is  well  known  that  plants  grown  in 


INFLUENCE  OF  ENVIRONMENT  349 

poor  soil  are  smaller  and  produce  smaller  seeds 
than  those  grown  in  good  soil,  and  deVries, 
Baur  and  Harris  find  that  such  seeds  produce 
smaller  plants  with  smaller  seeds  than  do  seeds 
of  normal  size.  This  is  an  after  effect  of  poor 
nutrition  which  changes  the  amount  of  food 
material  in  the  seeds  and  through  this  the  size 
of  the  plant  which  develops  from  the  seed,  but 
it  does  not  change  the  hereditary  constitution. 
Woltereck  found  that  in  Daphnia  there  is  an 
after  effect  of  cold  lasting  for  one  or  two  gen- 
erations, and  this  he  calls  "induction"  when  the 
effect  lasts  for  one  generation,  or  "preinduc- 
tion"  when  it  lasts  for  two  or  three  genera- 
tions. Whitney  found  that  rotifers  poisoned 
with  alcohol  were  weaker  in  resistance  to  cop- 
per salts  and  were  less  fertile  than  others,  and 
when  brought  back  to  normal  conditions  the 
first  generation  was  weak  but  the  second  was 
normal.  In  man  also  alcohol  may  have  an  in- 
duction effect  on  offspring,  but  it  does  not  seem 
to  alter  hereditary  constitution.  Probably  of 
a  similar  character  are  Sumner's  results;  he 
found  that  mice  raised  in  the  cold  have  shorter 
tails  than  those  raised  at  higher  temperatures 


350  HEREDITY  AND  ENVIRONMENT 

and  this  modified  character  appears  in  the 
next  generation.  If  this  is  an  after  effect  or 
"induction"  it  should  disappear  in  the  follow- 
ing generations. 

Kammerer  found  that  black-  and  yellow- 
spotted  salamanders,  reared  on  yellow  soil 
gradually  lose  their  black  color  becoming  more 
yellow,  and  their  young  continue  to  grow  more 
yellow  until,  finally  almost  all  black  may  dis- 
appear. The  offspring  of  such  salamanders 
are  more  yellow  than  normal ;  but  this  also  may 
be  an  after  effect  or  "induction"  which  would 
soon  disappear  under  usual  conditions. 

Probably  such  cases  are  not  instances  of  true 
inheritance ;  they  do  not  signify  a  change  in  the 
hereditary  constitution  but  an  influence  on  the 
germ  cells  of  a  nutritive  or  chemical  sort  com- 
parable with  what  takes  place  when  fat  stains 
are  fed  to  animals;  the  eggs  of  such  animals 
are  stained  and  the  young  which  develop  from 
such  eggs  are  also  stained,  though  the  germi- 
nal constitution  remains  unchanged.  The 
very  fact  that  the  changed  condition  is  reversi- 
ble and  that  it  disappears  within  a  short  time 
is  evidence  that  it  is  not  really  inherited. 


INFLUENCE  OF  ENVIRONMENT  351 

In  conclusion:  (1)  Developed  characters, 
whether  "acquired"  or  not,  are  never  transmit- 
ted by  heredity,  and  the  hereditary  constitu- 
tion of  the  germ  is  not  changed  by  changes  in 
such  characters.  (2)  Probably  environmental 
stimuli  acting  upon  germ  cells  at  an  early 
stage  in  their  development  may  rarely  cause 
changes  in  their  hereditary  constitution,  but 
changes  produced  in  somatic  cells  do  not 
cause  corresponding  changes  in  the  hereditary 
constitution  of  the  germ  cells.  (3)  Germ  cells 
like  somatic  cells  may  undergo  modifiations 
which  are  not  hereditary ;  they  may  be  stained 
with  fat  stains  and  the  generation  to  which 
they  give  rise  be  similarly  stained;  they  may 
be  poisoned  with  alcohol  or  modified  by  tem- 
perature and  such  influence  be  carried  over  to 
the  next  generation  without  becoming  heredi- 
tary. All  such  cases  are  known  as  "induction" 
and  many  instances  of  the  supposed  inheri- 
tance of  acquired  characters  come  under  this 
category. 


352  HEREDITY  AND  ENVIRONMENT 


E.  APPLICATIONS    TO    HUMAN    DEVELOP- 
MENT:   EUTHENICS 

Man's  environment  is  more  extensive  than 
that  of  any  other  animal,  and  its  influence  on 
his  development  is  correspondingly  greater. 
In  addition  to  chemical  and  physical  stimuli 
which  are  potent  factors  of  development  in 
the  case  of  all  organisms,  man  lives  in  a  world 
of  psychical,  social  and  moral  stimuli  which 
exert  a  profound  influence  on  him.  He  is 
stimulated  not  merely  by  present  environment 
but  also  by  memories  of  past  experiences  and 
anticipations  of  future  ones.  Through  intelli- 
gence and  social  cooperation  he  is  able  to  con- 
trol environment  for  particular  ends,  in  a 
manner  quite  impossible  in  other  organisms. 
On  the  other  hand  heredity  is  no  more  power- 
ful as  a  factor  of  development  in  the  case  of 
man  than  in  any  other  organism.  Conse- 
quently the  relative  importance  of  heredity  and 
environment  is  not  the  same  in  the  development 
of  an  intelligent  and  social  being,  like  man  of 


INFLUENCE  OF   ENVIRONMENT  353 

the  present  age,  as  it  is  in  lower  organisms. 
For  man  and  for  every  other  living  creature 
heredity  fixes  the  possibilities  of  development, 
it  "sets  bounds  about  us  which  we  cannot 
pass";  but  the  more  complex  those  possibilities 
become  the  more  complex  must  be  the  environ- 
ment which  calls  them  forth  and  the  more 
varied  become  the  results  of  development  un- 
der altered  conditions  of  life. 

Functional  activity  also  plays  a  larger  part 
in  man's  development  than  in  that  of  any  other 
animal,  owing  to  the  longer  period  of  his 
development  and  to  the  more  extensive  and 
varied  training  which  he  is  capable  of  under- 
going. It  is  a  notable  fact  that  the  period  of 
immaturity  in  man  is  longer  than  in  any  other 
animal,  and  it  is  during  this  formative  period 
that  environment  and  education  have  their 
greatest  influence.  Other  animals  develop 
much  more  rapidly  than  man  but  that  develop- 
ment sooner  comes  to  an  end.  The  children  of 
lower  races  of  man  develop  more  rapidly  than 
those  of  higher  races  but  in  such  cases  they 
also  cease  to  develop  at  an  earlier  age.  The 
prolongation  of  the  period  of  infancy  and  of 


354  HEREDITY  AND  ENVIRONMENT 

immaturity  in  the  human  race  greatly  in- 
creases the  importance  of  environment  and 
training  as  factors  of  development. 

The  possible  training  of  human  faculties  is 
also  more  varied  and  extensive  than  in  other 
animals,  not  only  because  those  faculties  are 
more  numerous  but  also  because  they  are 
more  plastic  and  are  capable  of  higher  de- 
velopment, that  is,  are  more  educable.  Hu- 
man faculties  are  functions  and  methods  of 
reaction,  which  are  dependent  in  part  upon  the 
bodily  mechanism  and  in  part  upon  environ- 
ment and  training.  Habits  are  the  usual 
methods  of .  responding  to  stimuli,  and  they 
may  be  classified  as  inherent  or  acquired.  The 
latter  are  in  a  sense  forced  upon  organ- 
isms by  environmental  conditions.  All  educa- 
tion is  habit  formation,  and  good  education 
like  good  environment  is  such  an  experience  as 
leads  to  the  formation  of  good  bodily,  intel- 
lectual, social  and  moral  habits;  it  consists  in 
placing  the  individual  in  such  an  environment 
and  bringing  such  stimuli  to  bear  upon  him  as 
to  call  forth  desirable  responses  and  to  sup- 
press undesirable  ones. 


INFLUENCE  OF  ENVIRONMENT  355 

Only  that  environment  and  training  is  good 
which  leads  to  the  development  of  good  habits 
and  traits  and  to  the  suppression  of  bad  ones. 
What  we  commonly  call  "good  environment" 
is  frequently  the  worst  possible,  what  is  often 
called  a  bad  environment  may  be  the  best  pos- 
sible. We  are  all  strangely  blind  with  regard 
to  these  matters.  We  know  of  many  cases  in 
which  men  began  their  careers  on  a  farm,  in 
the  backwoods,  on  a  flat-boat,  amidst  hard- 
ships and  discomforts  of  every  sort  and  yet 
who  achieved  great  distinction.  And  we 
speak  of  such  men  as  winning  in  spite  of  dis- 
advantages, forgetting  that  often  these  very 
disadvantages,  hardships,  discomforts,  have 
proved  stimuli  which  have  given  them  sturdy 
bodies,  good  judgments,  good  morals,  and 
have  called  forth  all  their  best  qualities.  On 
the  other  hand  under  different  circumstances 
or  with  different  men  such  conditions  may 
prove  to  be  too  hard,  too  severe,  and  the  result 
be  disastrous.  But  environment  may  be  too 
good  as  well  as  too  hard.  Food  may  be  too 
rich  and  too  abundant  for  good  health,  life 
may  be  too  easy  and  luxurious  for  the  develop- 


356  HEREDITY  AND  ENVIRONMENT 

merit  of  character.  Luxury,  easy  lives,  refined 
surroundings  have  less  of  educational  value 
than  we  commonly  suppose  and  they  may  be 
a  positive  menace.  That  environment  is  bad, 
however  cultured,  refined  or  pleasant  it  may 
be,  which  leads  to  the  development  of  bad 
traits  of  body  or  of  mind.  In  general  the  best 
environment  is  one  which  avoids  extremes,  one 
which  is  neither  too  easy  nor  too  hard, — one 
which  produces  maximum  efficiency  of  body 
and  of  mind. 

In  education  also  we  are  strangely  blind  to 
proper  aims  and  methods.  Any  education  is 
bad  which  leads  to  the  formation  of  habits  of 
idleness,  carelessness,  failure,  instead  of  habits 
of  industry,  thoroughness  and  success.  Any 
religious  or  social  institution  is  bad  which 
leads  to  habits  of  pious  make-believe,  insin- 
cerity, slavish  regard  for  authority  and 
disregard  for  evidence,  instead  of  habits  of  sin- 
cerity, open  mindedness  and  independence. 

Frequently  the  training  of  the  human  being, 
like  the  training  of  a  star-fish,  consists  in  limit- 
ing his  activities  to  particular  lines.  Some 
physical  defect  which  prevented  a  child  from 


INFLUENCE  OF   ENVIRONMENT  357 

engaging  in  the  usual  activities  of  children  has 
often  turned  his  attention  to  scholarship. 
Galton  says  that  great  divines  have  usually 
had  very  poor  health.  Genius  is  frequently 
associated  with  physical  defects.  Great  spe- 
cialization is  associated  with  corresponding 
limitations  in  other  directions.  Society  needs 
the  genius  and  the  specialist,  but  for  the  gen- 
eral good  of  mankind  the  generalized  type  of 
man  is  needed  even  more  than  the  specialized. 

No  given  environment  or  training  can  be 
good  for  every  individual,  nor  for  the  same 
individual  at  every  stage  of  development. 
Every  individual  is  unique  and  if  the  best  re- 
sults are  to  be  had  must  have  unique  environ- 
ment and  training,  which  must  be  supplied  by 
omniscient  intelligence.  The  impossibility  of 
securing  the  absolutely  best  conditions  of  de- 
velopment need  not  prevent  society  from  se- 
curing better  conditions  than  those  which  now 
prevail. 

It  is  plain  that  environment  and  education 
play  a  greater  part  in  the  development  of  man 
than  in  that  of  other  animals,  whereas  heredity 
plays  the  same  part;  but  it  is  difficult  if  not 


358  HEREDITY  AND  ENVIRONMENT 

impossible  to  determine  the  relative  impor- 
tance of  these  three  factors.  So  far  as  intellect 
and  morals  are  concerned  we  are  all  inclined 
to  place  greater  weight  upon  the  extrinsic  than 
upon  the  intrinsic  factors,  but  this  opinion  is 
not  based  upon  scientific  evidence.  So  far  as 
organisms  below  man  are  concerned  there  is 
general  agreement  that  heredity  is  the  most 
important  factor,  and  this  opinion  is  held  also 
for  man  by  those  who  have  made  a  thorough 
study  of  heredity.  Galton  has  made  the  best 
scientific  study  of  this  subject  in  the  case  of 
identical  twins,  in  which  as  we  know  heredity 
is  the  same  in  the  two,  both  individuals  having 
come  from  the  same  oosperm  (Fig.  76).  In 
bodily  and  mental  characters  such  twins  are  re- 
markably alike ;  the  differences  which  exist  are 
slight  and  may  usually  be  traced  to  different 
environmental  and  educational  influences, 
and  particularly  to  different  illnesses.  Galton 
sums  up  his  study  with  these  words:  "There 
is  no  escape  from  the  conclusion  that  nature 
prevails  enormously  over  nurture  when  the 
differences  of  nurture  do  not  exceed  what  is 
commonly  to  be  found  among  persons  of 


INFLUENCE  OF   ENVIRONMENT  359 

the   same  rank  of  society   and  in  the   same 
country." 

The  part  played  by  these  different  factors 
of  development  may  be  graphically  illustrated 
by  the  accompanying  diagram  (Fig.  83),  in 
which  the  base  line  represents  heredity  and 
the  other  lines  represent  the  extrinsic  factors 
of  environment  and  education.  For  each  in- 
dividual heredity  is  a  constant  factor  but  en- 
vironment and  training  are  variables.  With 


Heredity 

FIG.  83.  Diagram  to  show  the  influence  of  heredity,  en- 
vironment and  training  in  the  development  of  an  individual. 
Various  types  of  individuals  (represented  by  the  triangles) 
may  be  produced  from  the  same  germ  cells  (heredity)  if  the 
environment  and  training  are  variable. 


360  HEREDITY  AND  ENVIRONMENT 

a  given  heredity  the  characteristics  of  the 
developed  organism  may  vary  enormously 
depending  upon  the  extrinsic  factors.  He- 
reditary possibilities  are  not  changed  by  acci- 
dents of  environment  but  development  is  so 
changed.  After  the  fertilization  of  the  egg  the 
hereditary  potencies  of  every  organism  are  un- 
alterably fixed  but  the  extrinsic  factors  re- 
main variable  and  may  be  controlled. 

All  of  our  social  and  ethical  institutions  such 
as  government,  education  and  religion  deal 
only  with  extrinsic  factors  of  development  and 
of  life.  Nevertheless  there  is  no  evidence 
that  such  extrinsic  influences  ever  modify  he- 
redity, no  evidence  that  the  effects  of  good 
environment  or  good  training  ever  change  the 
germinal  constitution.  The  influences  of 
environment  and  education  affect  only  the  de- 
velopment of  the  individual  and  not  the  con- 
stitution of  the  race,  and  consequently  such  in- 
fluences are  temporary  in  effect  and  must  be 
repeated  generation  after  generation. 

But  though  the  effects  of  environment  and 
training  are  not  inherited,  the  environment 
and  training  and  experience  of  former  gener- 


INFLUENCE  OF  ENVIRONMENT  361 

ations  are  handed  down  to  later  generations 
through  custom,  tradition,  history.  We  do  not 
inherit  through  the  germ  cells  the  effects  on 
our  ancestors  of  their  training  and  environ- 
ment, but  we  do  inherit,  in  the  property  sense 
of  that  word,  their  environment,  customs,  in- 
stitutions. In  short  the  experiences  and  ac- 
complishments of  past  generations  are  not 
inherited  through  the  germ  cells  but  are  inher- 
ited through  society.  In  this  sense  "We  are 
the  heirs  of  all  the  ages." 

Because  of  this  social  inheritance  the  ex- 
trinsic conditions  of  life  continue  to  grow  more 
complex  age  after  age,  while  our  inherited  na- 
tures remain  unchanged.  All  moralists,  all 
religions,  have  recognized  the  very  general  ex- 
perience among  men  of  a  sense  of  imperfec- 
tion and  of  disharmony  with  social  and  ethical 
standards.  Huxley  held  that  the  spirit  of 
ethics  was  opposed  to  the  spirit  of  evolution. 
Metchnikoff  finds  these  disharmonies  due  to 
the  survival  of  bestial  instincts  in  man.  Galton 
finds  this  sense  of  sin  to  be  due  to  the  fact 
that  the  development  of  our  inherited  nature 
has  not  kept  pace  with  the  development  of  our 


362  HEREDITY  AND  ENVIRONMENT 

moral  civilization.  Our  psychical,  social  and 
moral  environment  has  come  to  us  from 
the  past  with  ever-increasing  increments,  every 
age  standing  on  the  shoulders  of  the  preceding 
one.  The  aspirations,  impulses,  responsibili- 
ties of  modern  life  have  become  enormous  and 
our  inherited  natures  and  abilities  have  not 
essentially  improved.  Social  heredity  has  out- 
run germinal  heredity  and  the  intellectual,  so- 
cial and  moral  responsibilities  of  our  times  are 
too  great  for  many  men.  Civilization  is  a 
strenuous  affair,  with  impulses  and  compul- 
sions which  are  difficult  for  the  primitive  man 
to  fulfil,  and  many  of  us  are  hereditarily  prim- 
itive men.  The  frequent  result  is  disharmony, 
poor  adjustment,  the  struggle  of  the  primitive 
instincts  with  high  ideals,  with  a  resulting 
sense  of  discouragement  and  defeat  which  of- 
ten ends  in  abnormal  states  of  mind.  The 
great  growth  of  alcoholism,  depravity  and  in- 
sanity is  an  ever-increasing  protest  and  menace 
of  weak  men  against  high  civilization.  We  are 
approaching  the  time  when  one  or  the  other 
must  give  way,  either  the  responsibilities  of 
life  must  be  reduced  and  the  march  of  civiliza- 


INFLUENCE  OF   ENVIRONMENT  363 

tion  stayed,   or   a  better  race  of  men,  with 
greater  hereditary  abilities,  must  be  bred. 

The  present  world-war,  with  its  appeal  to 
the  primitive  instincts  of  men  and  its  destruc- 
tion of  the  fairest  and  best  fruits  of  civilization, 
points  one  way  out  of  this  disharmony  between 
germinal  and  social  inheritance ;  but  it.  is  a  way 
from  which  all  sane  and  sober  minds  recoil. 
Wars  and  revolutions  shake  off  the  burdens  of 
social  inheritance  but  they  destroy  the  good 
along  with  the  bad  and  afford  only  local  and 
temporary  relief.  Mankind  cannot  return 
permanently  to  barbarism  or  savagery;  civili- 
zation must  be  and  will  be  preserved;  but  if 
society  is  to  really  advance  from  age  to  age  the 
natures  of  men  must  improve  as  well  as  their 
environment. 


CHAPTER  V 

CONTROL  OF  HEREDITY 
EUGENICS 


CHAPTER  V 

CONTROL  OF  HEREDITY:     EUGENICS 

It  is  the  aim  of  science  to  interpret  phe- 
nomena and  as  far  as  possible  to  control 
them.  To  what  extent  is  it  possible  to  control 
heredity  and  thus  to  improve  the  race,  as  well 
as  the  individual? 

A.  DOMESTIC  ANIMALS  AND  CULTIVATED 
PLANTS 

The  history  of  domesticated  animals  and 
cultivated  plants  shows  that  it  is  possible  to 
control  or  rather  guide  phenomena  of  he- 
redity and  evolution.  Very  many  species  of 
wild  animals  have  been  tamed  by  man  but 
only  about  40  species  may  be  classed  as 
domesticated.  In  a  number  of  instances  the 
wild  stocks  from  which  these  domestic  forms 
came  are  known  and  it  is  possible  to  compare 
them  with  their  modified  descendants  and  thus 

367 


368  HEREDITY  AND  ENVIRONMENT 

to  determine  the  degree  of  change  which  has 
been  brought  about  under  human  guidance. 
In  other  cases  where  the  original  wild  species 
are  unknown  it  is  possible  to  determine  the 
amount  of  modification  which  has  taken  place 
within  relatively  recent  times.  DeCandolle 
recognizes  247  species  of  cultivated  plants,  193 
of  which  still  exist  in  the  wild  state. 

The  degree  of  change  which  has  taken  place 
under  human  guidance  is  very  remarkable. 
In  some  cases  dozens  and  even  hundreds  of 
races  have  been  formed,  showing  the  most  re- 
markable differences  in  size,  structure  -and 
proportions  of  parts,  as  well  as  in  functions, 
instincts  and  behavior.  The  extent  to  which 
heredity  may  be  guided  by  man  is  forcibly  il- 
lustrated by  our  present  races  of  domestic 
pigeons  which  Darwin  said  would  be  classed 
by  any  naturalist  who  did  not  know  their  ori- 
gin in  not  less  than  twenty  different  species 
and  three  different  genera,  though  all  of  them 
have  descended  from  the  wild  rock  pigeon 
(Figs.  84,  85)  ;  or  by  the  numerous  races  of 
dogs  varying  in  size  from  a  toy  dog  to  a  Great 
Dane  or  St.  Bernard  and  showing  almost  un- 


CONTROL  OF   HEREDITY:     EUGENICS  369 


FIG.  84.  Races  of  Domestic  Pigeons,  the  wild  rock  pigeon 
being  shown  in  the  center.  (From  Romanes,  "Darwin  and 
After  Darwin.") 


3TO  HEREDITY  AND  ENVIRONMENT 


FIG.  85.     Races    of    Domestic    Pigeons,    continued.      (From 
Romanes.) 


CONTROL  OF   HEREDITY:     EUGENICS  371 


FIG.  86.     Races  of  Fowls.      (From   Romanes.) 


372 


HEREDITY  AND  ENVIRONMENT 


FIG.  87.     Races  of  Fowls,  continued.     (From  Romanes.) 


CONTROL  OF  HEREDITY:     EUGENICS  373 

believable  differences  in  structure  and  dispo- 
sition; or  by  the  great  variety  of  domestic 
fowls,  all  of  which  have  probably  descended 
from  the  wild  jungle-fowl;  or  by  modern  races 
of  horses,  cattle,  sheep,  or  swine.  These  are 
only  a  few  of  the  many  illustrations  which 
could  be  cited  of  the  practical  control  of  he- 
redity and  evolution  for  human  purposes. 
How  have  the  present  races  of  domesticated 
animals  and  cultivated  plants  been  produced? 


FIG.  88.     Wild    Boar    contrasted    with    a    modern    domestic 
Pig.      (From   Romanes.) 


374  HEREDITY  AND  ENVIRONMENT 


FIG.  89.     Different      Breeds     of     British     Cattle.        (Fror 
Romanes.) 


CONTROL  OF  HEREDITY:     EUGENICS          375 

I.  THE  INFLUENCE  or  ENVIRONMENT  IN  PRO- 
DUCING NEW  RACES 

There  is  a  popular  belief  that  the  value  of 
cultivated  races  is  due  to  good  environment, 
good  food,  good  soil,  protection  from  enemies, 
etc.,  and  that  if  turned  out  to  shift  for  them- 
selves they  revert  at  once  to  the  original  wild 
stock.  There  are  two  ways  in  which  it  is  con- 
ceivable that  new  races  might  be  produced  by 
environmental  influences : 

1.  By  the  direct  inheritance  of  somatic  or 
personal  characters  acquired  under  the  stimu- 
lus of  the  environment.     In  spite  of  popular 
opinion  in  favor  of  this  view  there  is  no  evi- 
dence that  this  ever  occurs.    There  is  no  doubt 
that  environment  has  much  to  do  with  indi- 
vidual development,  but  it  does  not  usually 
modify  the  hereditary  constitution  of  the  race. 

2.  It  is  possible  that  environmental  changes 
acting  upon  germ  cells  at  a  sensitive  period  of 
their  development  may  produce  germinal  vari- 
ations or  mutations  and  thus  give  rise  to  new 
races.    There  is  considerable  evidence  that  en- 


376  HEREDITY  AND  ENVIRONMENT 

vironment  does  sometimes  act  in  this  way, 
but  there  is  no  evidence  that  such  changes  in 
hereditary  constitution  are  reversible  and  that 
the  race  reverts  to  its  former  state  when  the 
old  environment  is  restored.  Such  reversible 
changes  undoubtedly  occur  in  somatic  char- 
acters but  they  are  not  inherited;  they  are 
modifications  of  development,  not  of  heredity ; 
they  are  personal  fluctuations,  not  racial 
mutations. 

II.  ARTIFICIAL,  SELECTION 

Since  the  beginning  of  historic  times,  and 
probably  through  long  prehistoric  ages,  breed- 
ers have  followed  the  method  of  selecting  de- 
sirable individuals  for  propagating  their  stock. 
There  can  be  no  doubt  that  almost  all  that  man 
has  done  in  the  production  of  domestic  ani- 
mals and  cultivated  plants  has  been  accom- 
plished by  this  process  of  selection. 

1.  The  methods  of  selection  have  varied  con- 
siderably with  different  breeders;  sometimes 
they  have  selected: 

(a)  The  best  individuals  grown  in  the  best 
environment. 


CONTROL  OF  HEREDITY:   EUGENICS       37? 

(b)  The    best   individuals   grown   in   poor 
environment. 

(c)  Any  individual  of  good  pedigree,  irre- 
spective of  the  conditions  of  its  environment. 
The  fact  that  the  good  qualities  of  a  race  may 
be  maintained  by  this  last  method  as  well  as 
by  either  of  the  others  shows  how  little  good 
or  bad  environment  has  to  do  with  hereditary 
or  racial  characters. 

2.  How  has  selection  acted? 

(a)  Until  very  recently  it  was  generally 
believed  that  continued  selection  of  individuals 
which  showed  desirable  characters  gradually 
led  to  the  improvement  of  those  characters  and 
thus  to  the  production  of  new  races;  it  was 
supposed  that  the  character  in  question  was 
"built  up"  by  continued  selection  in  one  direc- 
tion, and  that  the  average  development  of  the 
character  in   all  the   offspring  was   thus  in- 
creased in  successive  generations.    It  was  this 
view  as  to  the  supposed  action  of  artificial  se- 
lection which  formed  the  basis  of  Darwin's 
theory  of  natural  selection. 

(b)  The    brilliant    researches    of    deVries, 
Johannsen,  Jennings,  Tower  and  several  oth- 


378  HEREDITY  AND  ENVIRONMENT 

ers  have  shown  that  selection  creates  nothing 
new  but  merely  isolates  mutants  or  distinct 
hereditary  lines  which  are  already  present  in  a 
mixed  population.  Johannsen  found  that  from 
a  single  species  of  beans  he  was  able,  by  keep- 
ing the  progeny  of  each  individual  bean  sepa- 
rate from  the  others,  to  get  19  different  "pure 
lines"  of  beans,  each  differing  in  certain  re- 
spects from  every  other  line.  He  further  found 
that  when  extremely  large  or  small  individuals 
from  any  pure  line  were  selected  and  bred 
from,  none  of  the  progeny  showed  that  char- 
acter in  a  still  more  extreme  degree  but  all 
merely  fluctuated  within  the  original  extremes 
of  that  line.  In  short,  he  concludes  that  selec- 
tion within  a  pure  line  is  absolutely  without 
effect  in  modifying  any  character  in  the  off- 
spring of  that  line. 

Jennings  found  that  different  races  or  pure 
lines  of  Paramecium  differ  in  size,  structure 
and  rate  of  division,  and  that  these  differences 
are  "as  rigid  as  iron."  With  respect  to  aver- 
age length  of  body  he  was  able  to  isolate  eight 
pure  lines  which  constantly  differed  more  or 
less  from  one  another.  Within  each  of  these 


CONTROL  OF  HEREDITY:   EUGENICS       379 

lines  there  was  considerable  fluctuation  in  size, 
but  he  was  unable  by  selecting  extremes  to 
increase  these  fluctuations,  the  progeny  of  any 
pure  line  always  fluctuating  about  the  mean 
of  that  line  (Fig.  44). 

Similarly  Tower  found  in  his  studies  on  the 
potato  beetle  that  he  was  unable  to  shift  the 
mean  or  the  extremes  of  any  character  by  se- 
lection of  extreme  forms  of  an  inbred  line. 

Pearl  also  made  an  extensive  study  of  the 
records  of  breeding  experiments  extending 
over  many  years  in  which  the  attempt  was 
made  to  increase  the  egg-laying  capacity  of 
hens  by  selecting  in  each  generation  for  breed- 
ing only  those  which  had  a  high  record  for 
egg  production.  It  was  found  that  certain 
"blood  lines"  produced  a  larger  number  of 
eggs  than  other  lines,  but  by  no  amount  of 
selection  was  it  possible  to  increase  the  egg 
production  within  any  line. 

These  and  other  experiments  seem  to  de- 
monstrate the  absolute  inability  of  selection  to 
produce  anything  new  or  to  improve  any  char- 
acter of  a  pure  race.  Nevertheless  selection  is 
of  great  service  in  separating  good  lines  or 


380  HEREDITY  AND  ENVIRONMENT 

races  from  poor  ones,  and  this  is  the  whole 
rationale  of  the  artificial  selection  practiced  by 
breeders. 

The  elimination  of  certain  races  by  natural 
selection  may  be  an  important  factor  in  evo- 
lution though  it  has  nothing  to  do  with  the 
formation  of  new  characters  or  new  races  but 
serves  merely  as  a  sieve,  as  deVries  has  ex- 
pressed it,  to  sort  the  individuals  which  are 
supplied  to  it.  Selection  has  no  power  to 
make  or  change  characters,  but  it  may  pre- 
serve certain  lines  and  eliminate  others  and 
thus  fix  the  type  of  a  species.  Finally  the 
elimination  of  the  unfit  by  natural  selection  is 
still  the  only  natural  explanation  of  fitness,  or 
adaptation,  in  organisms. 

III.  METHODS  or  MODERN  GENETICS 

1.  MendeUan  Association  and  Dissociation 
of  Characters. — Breeders  have  long  known 
that  it  is  possible  to  get  certain  desirable  char- 
acters of  an  organism  from  one  race  and  other 
desirable  characters  from  another  race.  But 
since  the  discovery  of  the  Mendelian  principles 
of  heredity  such  new  combinations  of  old  char- 


CONTROL  OF  HEREDITY:     EUGENICS          381 

acters  have  been  made  repeatedly,  and  with 
almost  the  same  certainty  of  results  as  when 
the  chemist  makes  combinations  of  elements 
or  radicals. 

In  Fig.  90,  A  and  B,  are  shown  two 
guinea-pigs,  one  having  long  (L),  rough  and 
tumbled  (T),  white  (W)  hair,  and  the  other 
having  short  (S),  smooth  (Sm),  red  (R)  hair. 
When  such  animals  are  crossed  one  should  get 
in  the  F2  generation  64  genotypes  and  8 
phenotypes,  one  of  each  of  the  latter  being 
homozygous  and  breeding  true,  as  is  shown  ya 
Yig.  54  for  trihybrid  peas.  These  8  pheno- 
types of  this  cross  are  STR,  STW ',  SSmR, 
SSrnW,  LTR,  LTW,  LSmR,  LSmW.  In 
Figs.  90,  C  and  D,  and  91,  A,  B,  C,  are 
shown  5  of  these  8  phenotypes  which  were  ob- 
tained by  Castle  from  this  cross.  These  fig- 
ures well  illustrate  the  new  combinations  of 
Mendelian  characters  which  may  be  obtained 
by  cross  breeding. 

This  is  the  chief  method  employed  by  Bur- 
bank  in  producing  his  really  wonderful  "new 
creations  in  plant  life."  By  extensive  hybridi- 
zation he  brings  about  many  new  combinations 


384  HEREDITY  AND  ENVIRONMENT 

of  old  characters,  a  few  of  which  may  be  com- 
mercially valuable,  and  sometimes  actually 
hew  characters  or  mutations  appear,  possibly 
as  a  result  of  the  interaction  of  old  characters, 
or  rather  of  their  factors. 

One  of  the  striking  results  of  modern  work 
in  plant-breeding  has  been  the  discovery  of  the 
greatly  increased  vigor  of  certain  hybrids  as 
compared  with  either  pure-bred  parent.  In 
general  it  is  not  possible  to  tell  without  pre- 
vious experience  what  the  character  of  the 
hybrid  of  two  races  or  "lines"  will  be;  some- 
times it  is  more  and  sometimes  less  vigorous 
than  either  parent,  but  not  infrequently  it  is 
more  vigorous.  East  and  Shull  have  shown 
that  hybrids  between  two  races  of  corn  may 
be  very  much  larger  and  more  fertile  than 
either  parent.  In  some  instances  the  yield  of 
corn  per  acre  has  been  increased  from  20-30 
bushels  to  80-90  bushels,  and  in  one  case  to 
more  than  250  bushels  per  acre  (Figs.  92,  93) . 
Unfortunately  such  hybrid  races  of  corn  do 
not  continue  to  breed  true  and  the  crossing 
must  be  made  anew  in  each  generation  if 
maximum  results  are  to  be  had.  Nevertheless 


II 


§1 


1 1 

l-s 


CONTROL  OF   HEREDITY:     EUGENICS  387 

this  method  of  hybridization  or  "heterozygo- 
sis,"  as  it  has  been  called,  offers  an  extremely 
important  means  of  quickly  producing  very 
vigorous  and  fruitful  individuals,  but  not  lines 
or  races  which  breed  true. 

This  is  what  is  meant  by  the  control  of  he- 
redity, namely  the  possibility  of  preventing 
the  reproduction  of  individuals  with  bad  traits, 
and  of  making  new  and  favorable  combina- 
tions of  old  traits  by  means  of  selective 
breeding,  and  of  seizing  upon  and  perpetuat- 
ing new  and  favorable  mutations. 

2.  Origin  of  Mutations. — Mendelian  asso- 
ciation and  dissociation  of  characters  produces 
new  forms  of  adult  animals  and  plants  but  not 
new  hereditary  characters.  Permutations  of 
Mendelian  characters  we  may  have  almost 
without  number,  of  new  combinations  of  these 
there  may  be  no  end,  but  no  new  unit  charac- 
ters are  formed  by  such  temporary  combina- 
tions, no  new  inheritance  factors  are  created  or 
evolved.  New  combinations  of  factors  may 
be  compared  to  new  combinations  of  chemical 
elements;  you  can  always  get  out  of  the 
combination  what  went  into  it,  whereas  new 


388  HEREDITY  AND  ENVIRONMENT 

factors  are  comparable  to  the  changes  which 
take  place  in  certain  atoms,  for  example  ra- 
dium, by  which  the  element  itself  is  changed  in 
an  irreversible  manner.  Evolution  depends 
upon  the  appearance  of  new  characters;  the 
discoveries  of  Mendel  show  us  how  to  follow 
old  characters  through  many  combinations  and 
through  many  generations,  but  they  do  not 
show  us  how  new  characters  arise.  These  dis- 
coveries have  given  us  an  invaluable  method 
of  sorting  and  combining  hereditary  qualities, 
but  Mendelian  inheritance  as  such  does  not 
furnish  the  materials  for  evolution. 

The  actual  origin  of  new  hereditary  charac- 
ters or  mutations  is  obscure.  Practically  all 
of  the  earlier  workers  and  writers  on  evolution 
have  found  the  principal  causes  of  transmuta- 
tion in  the  action  of  extrinsic  or  environmental 
forces  on  the  organism.  As  the  result  of  years 
of  labor  on  this  subject  Darwin  concluded  that 
"variability  of  every,  sort  is  due  to  changed 
conditions  of  life";  but  in  the  light  of  modern 
genetics  such  a  statement  is  too  sweeping. 

It  is  well  known  that  environmental  changes 
produce  many  kinds  of  modifications  in  organ- 


CONTROL  OF   HEREDITY:     EUGENICS  389 

isms,  and  in  general  these  modifications  are  the 
more  profound  the  earlier  they  occur  in  onto- 
geny ;  it  is  known  that  slight  alterations  of  the 
germ  cells  may  produce  great  modifications  of 
adult  structure,  and  yet  one  of  the  most  strik- 
ing results  of  recent  work  is  to  show  the  small 
effect  of  environmental  changes  on  racial  char- 
acters. Marked  individual  modifications  may 
be  produced  which  do  not  become  racial. 
Usually  not  one  of  thousands  of  variations 
which  occur  has  any  evolutionary  value. 
These  fluctuations  come  with  changing  en- 
vironment and  with  changing  environment 
they  disappear.  Very  rarely  a  sudden 
variation  or  mutation  arises  which  is  per- 
petuated by  heredity  and  which  forms  the 
basis  of  a  new  race  (Figs.  94,  95).  In  most 
cases  such  mutations  consist  in  the  dropping 
out  of  some  old  character  rather  than  in  the 
addition  of  a  new  one,  but  at  least  they  repre- 
sent modifications  of  hereditary  constitution 
and  as  such  they  furnish  material  for  evolu- 
tion. Whence  and  how  they  appear  we  rarely 
know,  for  like  the  kingdom  of  heaven  they 
come  without  observation.  Their  infrequencv 


390  HEREDITY  AND  ENVIRONMENT 

amidst  the  multitude  of  fluctuations  indicates 
the  wonderful  stability  of  racial  types  and 
teaches  respect  for  Weismann's  doctrine  of  a 
germ  plasm  relatively  stable,  independent  and 
continuous. 


FIG.  94.  Common  Colorado  Potato  Beetle,  Leptinotarsa. 
a,  normal  undecemlineata,  b,  the  mutant  augusto-vittata,  c,  the 
mutant  melanothorax,  d,  normal  decemlineata,  e,  the  mutant 
tortuosa,  f,  the  mutant  defecto-punctata.  (From  Plate  after 
Tower.) 


CONTROL  OF  HEREDITY:     EUGENICS          391 

This  distinction  between  somatic  and  germi- 
nal variations,  between  those  which  concern 
only  the  individual  and  those  which  are  in- 
herited and  furnish  material  for  evolution, 
marks  the  greatest  advance  in  the  study  of 
evolution  since  the  work  of  Darwin.  And  just 
as  these  germinal  variations  are  the  only  ones 
of  importance  in  the  process  of  evolution  so 
the  question  of  their  origin  is  the  greatest  evo- 
lutionary problem  of  the  present  day.  How 
are  such  germinal  variations  produced? 

There  is  evidence,  as  was  pointed  out  in  the 
last  lecture,  that  environmental  changes  of  the 
right  sort  acting  upon  germ  cells  at  the  right 
stage  may  lead  to  permanent  modification  of 
the  hereditary  organization.  The  observa- 
tions of  Hensen  on  yeast  and  of  Tower  and 
Morgan  on  insects  strongly  support  this 
conclusion. 

It  is  probable  that  certain  changes  in  en- 
vironment acting  upon  germ  cells  at  the  time 
of  their  maturation  divisions  may  lead  to  new 
distributions  of  chromosomes  or  even  to 
changes  in  the  composition  of  individual  chro- 
mosomes, thus  producing  new  hereditary 


392  HEREDITY  AND  ENVIRONMENT 

types.  Certain  mutants  of  Oenothera  (Fig. 
95)  seem  to  be  of  this  sort;  for  example  O. 
lamarckiana  has  14  chromosomes,  O.  lata  15, 
O.  semi-gigas  21,  O.  gigas  28,  and  these  vari- 
ations in  the  number  of  chromosomes  are  prob- 
ably due  to  abnormalities  in  the  maturation 
divisions.  It  is  significant  that  the  mutants 
lata  and  semi-gigas  have  occurred  several 
times,  whereas  gigas  appeared  but  once;  this 
may  be  explained  by  the  fact  that  the  chances 
of  the  doubling  of  chromosomes  in  both  germ 
cells  (gigas)  are  very  few  compared  with  the 
chances  of  their  doubling  in  one  germ  cell 
(semi-gigas)  or  of  their  increase  by  one  in  one 
germ  cell  (lata). 

It  is  probable  that  mutations  are  not  always 
nor  even  usually  associated  with  changes  in 
the  number  of  chromosomes.  Where  the  num- 
ber of  chromosomes  remains  constant  the 
change  may  take  place  in  the  number  or  com- 
position of  the  chromomeres  or  units  of  the 
next  lower  order,  but  it  would  be  practically 
impossible  to  find  such  changes  in  bodies  so 
small  and  so  numerous.  Whatever  the  cellu- 
lar changes  may  be  which  accompany  muta- 


394  HEREDITY  AND  ENVIRONMENT 

tions,  it  is  certain  that  changes  take  place  in 
the  inheritance  factors.  Sometimes  factors 
drop  out,  as  in  the  white  sweet  peas  shown  in 
Fig.  57  and  in  many  other  cultivated  races 
of  plants  and  animals,  thus  producing  regres- 
sive mutations.  Indeed  most  of  our  domestic 
animals  and  cultivated  plants  have  arisen  by 
the  omission  of  something  which  their  wild 
ancestors  had.  Occasionally  new  factors  are 
added  thus  producing  progressive  mutations, 
which  are  of  special  importance  in  evolution. 

It  is  often  said  that  evolution  from  the 
amoeba  to  man  necessarily  involves  the  addi- 
tion of  many  new  inheritance  factors.  This  is 
probably  true,  but  the  addition  of  new  fac- 
tors does  not  mean  their  creation.  New  hered- 
itary factors  are  to  be  thought  of  as  we  think 
of  new  chemical  compounds,  which  are  formed 
by  new  combinations  of  the  same  old  elements, 
or  as  we  think  of  new  elements,  such  as  helium 
and  radium  emanation,  which  are  formed  by 
dissociation  of  radium.  As  compared  with 
chemical  elements  the  factors  of  heredity  are 
probably  very  complex  things  and  the  new  fac- 
tors which  appear  in  the  course  of  evolution 


CONTROL  OF  HEREDITY:     EUGENICS          395 

probably  arise  as  new  combinations  of  factors 
or  parts  of  factors  previously  present.  In  short 
as  modern  science  regards  all  types  of  organ- 
isms as  having  evolved  by  the  transformation 
of  previously  existing  organisms,  so  it  must  re- 
gard all  types  of  hereditary  factors  as  hav- 
ing existed  from  the  beginning  or  as  having 
evolved  by  transformations  of  preexisting  fac- 
tors; as  it  regards  all  types  of  chemical  com- 
pounds as  having  arisen  by  various  combina- 
tions of  chemical  elements,  so  it  must  regard 
all  "new"  elements  as  having  existed  from  the 
beginning  or  as  having  evolved  by  the  associa- 
tion or  dissociation  of  still  smaller  particles,  the 
negative  and  positive  electrons.  Nowhere  in 
the  entire  process  is  there  any  evidence  that 
factors  or  elements  or  electrons  are  created  de 
novo.  The  whole  process  is  one  of  evolution, 
that  is  of  new  combinations  of  existing  units, 
having  new  qualities  which  are  the  result  of 
these  new  combinations. 

If  these  changes  in  the  germ  plasm  may  be 
induced  by  extrinsic  conditions,  then  a  real 
experimental  evolution  will  be  possible ;  if  they 
cannot  be  so  induced  but  are  like  the  changes 


39(i  HEREDITY  AND  ENVIRONMENT 

taking  place  in  the  radium  atom  we  can  only 
look  on  while  the  evolutionary  processes  pro- 
ceed, selecting  here  and  there  a  product  which 
nature  gives  us  but  unable  to  initiate  or  con- 
trol these  processes. 

B.  CONTROL  OF  HUMAN  HEREDITY: 
EUGENICS 

I.  PAST  EVOLUTION  OF  MAN 

There  is  every  evidence  that  man  also,  no 
less  than  domesticated  animals,  has  evolved 
from  a  natural  or  wild  state.  The  most  primi- 
tive types  of  men  are  known  only  from  a  few 
fossil  remains,  which  indicate  that  these  primi- 
tive men  belonged  to  different  species,  and 
some  of  them  even  to  different  genera,  from 
Homo  sapiens  (Fig.  96).  Later  stages  in  the 
evolution  of  man  are  known  from  many  re- 
mains, implements  and  handiwork,  as  well  as 
from  certain  primitive  races  or  tribes  which 
have  persisted  to  the  present  time.  The  grades 
of  culture  represented  by  these  extinct  or  per- 
sistent tribes  and  by  modern  men  are  usually 
classified  as  savagery,  barbarism  and  civiliza- 


FIG.  96.  Extinct  Types  of  Man.  A  Skull  of  Homo  nean- 
derthalensis ;  from  Chapelle-aux-Saints,  France,  1908.  (After 
Boule).  B.  Skull  of  Pithecanthropus  erectus;  found  in  Trinil, 
Java,  1895;  parts  below  dotted  line  restored  by  J.  H. 
McGregor. 


398  HEREDITY  AND  ENVIRONMENT 

tion.  There  must  have  been  much  greater  evo- 
lution of  human  types  during  prehistoric  times 
than  since  the  beginnings  of  civilization.  The 
physical,  mental  and  moral  changes  which  took 
place  in  men  from  the  earliest  stages  of  sav- 
agery down  to  the  beginnings  of  civilization 
were  very  great,  but  they  were  nevertheless 
slight  compared  with  the  tremendous  changes 
which  must  have  occurred  in  those  long  ages 
before  the  ancestors  of  man  actually  became 
men.  Within  the  historic  period  the  evolution- 
ary changes  in  man  have  been  very  small. 
Minor  changes  have  occurred  and  are  still  go- 
ing on,  as  Osborn  has  shown  in  his  "Cartwright 
Lectures  on  Contemporary  Evolution  in 
Man,"  but  on  the  whole  the  race  has  remained 
relatively  stable  during  the  historic  epoch  as 
compared  with  the  much  longer  prehistoric  one. 
The  past  history  of  man  has  been  a  long  one, 
no  one  can  say  how  long,  but  probably  not  less 
than  a  million  years  have  passed  since  the 
genus  Homo  appeared,  and  not  less  than  one 
hundred  thousand  years  since  the  present 
species  arose.  There  is  every  reason  to  believe 
that  the  future  history  of  man  will  be  even 


CONTROL  OF   HEREDITY:     EUGENICS  399 

longer.  Barring  great  secular  changes,  catas- 
trophes or  cataclysms,  which  cannot  be  fore- 
seen or  provided  against,  man  controls  his 
own  destiny  on  this  planet. 

It  is  a  curious  fact  that  in  prescientific  times 
the  instability  of  nature  especially  appealed  to 
men.  How  often  in  the  past  have  men  looked 
forward  to  a  "speedy  end  of  the  world"!  It 
may  well  have  seemed  to  our  ancestors  a  use- 
less thing  to  take  any  thought  for  the  morrow 
if  very  soon  the  heavens  are  to  be  rolled  up  as 
a  parchment  and  the  elements  dissolved  in  fer- 
vent heat;  it  would  be  folly  to  plan  for  future 
ages  if  the  time  is  at  hand  when  the  angel 
shall  stand  with  one  foot  on  the  sea  and  the 
other  on  land  and  declare  that  time  shall  be 
no  more.  But  science  has  taught  us  something 
of  the  wonderful  stability  of  nature,  something 
of  the  immensity  of  past  time  and  of  future 
ages,  something  of  the  eternity  of  natural  pro- 
cesses. Compared  with  this  infinite  stability 
and  eternity  of  nature  what  are  our  little  sys- 
tems and  customs!  Our  years  and  centuries 
fall  like  grains  of  sand  into  this  abyss  of  time. 
Our  individual  lives  are  like  drops  of  water  in 


400  HEREDITY  AND  ENVIRONMENT 

the  great  oeean  of  life.  What  intellectual  de- 
velopment, what  social  institutions,  what  con- 
trol of  natural  processes  may  come  in  the  long 
ages  of  futurity  it  has  not  entered  into  the 
heart  of  man  to  conceive.  And  yet  so  far  as 
we  may  judge  by  the  small  portion  of  the 
record  of  the  past  which  we  can  read  there 
has  been  no  necessary  progress.  There  has 
been  "eternal  process  moving  on,"  but  not 
eternal  progress.  Stagnation,  degeneration, 
elimination,  as  well  as  progression,  have 
occurred  all  along  the  path  of  evolution. 
And  yet  on  the  whole  evolution  has  been 
progressive  and  there  is  no  reason  to  sup- 
pose that  the  elimination  of  the  unfit  and  the 
preservation  of  the  fit  will  cease  to  be  the 
law  of  future  evolution,  as  it  has  been  of  the 
past. 

There  are  four  principal  types  of  the  hu- 
man species — white,  yellow,  brown,  and 
black — and  many  subtypes  and  races.  These 
races  differ  in  many  regards  in  physical,  men- 
tal and  social  characteristics,  and  the  compar- 
ative value  of  these  races  has  frequently  been 
discussed.  It  is  difficult  to  take  an  impartial 


CONTROL  OF   HEREDITY:     EUGENICS  401 

view  of  such  a  matter,  though  I  suppose  there 
would  be  little  question  on  the  part  of  any 
well  informed  person  that  the  white  and  yel- 
low types  have  contributed  most  to  what  we 
call  civilization.  Nevertheless  every  race 
probably  has  good  qualities  which  could  be 
made  of  service  to  society.  The  various  races 
of  cattle,  horses,  sheep,  etc.,  are  all  useful  to 
man,  but  in  various  ways  and  degrees,  and  the 
same  is  true  of  various  races  of  men  with  re- 
spect to  civilization.  In  general  the  dominant 
races  are  the  most  capable  intellectually  and 
socially,  while  those  which  have  been  left  be- 
hind or  have  been  eliminated  have  been  the 
less  capable  races.  And  yet  some  very  good 
types,  possibly  with  capacities  for  high  social 
and  intellectual  development,  have  been  com- 
pletely exterminated,  as  for  instance  the  Luca- 
yan  Indians  of  the  West  Indies,  and  the 
aborigines  of  Tasmania. 

Few  animals  have  suffered  such  wholesale 
destruction  as  have  the  more  primitive  races 
of  men  in  different  parts  of  the  earth.  Several 
species  of  man  have  become  entirely  extinct, 
leaving  only,  as  is  generally  believed,  a  single 


402  HEREDITY  AND  ENVIRONMENT 

existing  species,  Homo  sapiens.  Race  exter- 
mination has  been  witnessed  in  relatively  re- 
cent times  and  on  a  large  scale  in  the  West 
Indies,  North  and  South  America,  Africa, 
Australia,  New  Zealand  and  the  Islands  of  the 
Pacific.  But  in  the  disappearance  of  native 
races  extermination  is  usually  supplemented 
by  amalgamation.  After  the  most  warlike 
members  of  a  race  have  been  destroyed  the 
more  peaceful  remnants  are  generally  incor- 
porated in  the  conquering  race.  Thus  the 
Maoris  of  New  Zealand,  the  finest  native  race 
with  which  the  English  have  come  in  contact  in 
their  colonies,  were  estimated  to  number  more 
than  a  quarter  of  a  million  at  the  end  of  the 
eighteenth  century.  Owing  to  destructive 
wars  among  the  tribes  and  with  the  English 
there  are  not  fifty  thousand  of  them  today, 
and  these  are  being  gradually  absorbed  into 
the  white  race. 

From  the  way  in  which  primitive  races  have 
gone  down  before  more  cultured  ones  there  is 
reason  to  believe  that  in  general  the  principle 
of  the  elimination  of  the  unfit  and  the  sur- 
vival of  the  fit  has  characterized  human  evolu- 


CONTROL  OF   HEREDITY:     EUGENICS  403 

tion  no  less  than  that  of  other  organisms. 
Undoubtedly  intelligence  has  played  a  great 
part  in  the  evolution  of  man  as  is  at  once 
apparent  when  we  consider  the  infinitely 
varied  experiments  by  which  he  has  worked 
his  way  from  savagery  to  civilization.  And  yet 
he  has  not  consciously  set  before  himself  an 
evolutionary  goal  to  be  attained  by  intelligent 
attention  to  principles  of  good  breeding. 


II.  CAN  HUMAN  EVOLUTION  BE  CONTROLLED? 

All  that  man  now  is  he  has  come  to  be  with- 
out conscious  human  guidance.  If  evolution 
has  progressed  from  the  amoeba  to  man  with- 
out human  interference,  if  the  great  progress 
from  ape-like  men  to  the  most  highly  civilized 
races  has  taken  place  without  conscious  hu- 
man control,  the  question  may  well  be  asked, 
Is  it  possible  to  improve  on  the  natural  method 
of  evolution?  It  may  not  be  possible  to  im- 
prove on  the  method  of  evolution  and 
yet  by  intelligent  action  it  may  be  possible  to 
facilitate  that  method.  Man  cannot  change  a 
single  law  of  nature  but  he  can  put  himself 


404  HEREDITY  AND  ENVIRONMENT 

into  such  relations  to  natural  laws  that  he  can 
profit  by  them. 

1.  Selective  Breeding  the  only  Method  of 
Improving  the  Race. — It  is  surely  not  possible 
to  improve  on  nature's  method  of  eliminating 
the  worst  lines  from  reproduction.  This  has 
been  the  chief  factor  in  the  establishment  of 
races  of  domesticated  animals  and  cultivated 
plants,  though  as  we  have  seen  it  has  probably 
had  nothing  to  do  with  the  origin  of  mutations. 
The  history  of  such  races  shows  that  evolution 
may  be  guided  to  human  advantage  by  intelli- 
gent elimination  and  selection,  and  probably 
any  hereditary  improvement  of  the  human  race 
must  be  accomplished  by  this  means,  though  of 
course  such  elimination  and  selection  can  ap- 
ply only  to  the  function  of  reproduction.  The 
method  of  evolution  by  the  elimination  of  per- 
sons, the  destruction  of  the  weak  and  cowardly 
and  antisocial,  which  was  the  method  practiced 
in  ancient  Sparta,  is  repugnant  to  the  moral 
sense  of  enlightened  men  and  cannot  be  al- 
lowed to  act  as  in  the  past;  but  the  worst 
types  of  mankind  may  be  prevented  from  pro- 
pagating, and  the  best  types  may  be  encour- 


CONTROL  OF   HEREDITY:     EUGENICS  405 

aged  to  increase  and  multiply.  This  is 
apparently  the  only  way  in  which  we  may 
hope  to  improve  permanently  the  human 
breed. 

2.  No  Improvement  in  Human  Heredity 
within  Historic  Times. — The  improvement  of 
environment  and  opportunities  for  individual 
development  enables  men  at  the  present  day 
to  get  more  out  of  their  heredity  than  was 
possible  in  the  past.  "The  advance  of  civiliza- 
tion" has  meant  only  improvement  of  environ- 
ment. But  neither  environment  nor  training 
have  changed  the  hereditary  capacities  of  man. 
There  has  been  no  perceptible  improvement  in 
human  heredity  within  historic  times,  nothing 
comparable  with  the  changes  which  have  oc- 
curred in  domesticated  animals.  Indeed  no 
modern  race  of  men  is  the  equal  of  certain  an- 
cient ones.  Galton  has  pointed  out  the  fact 
that  in  the  little  country  of  Attica  in  the  cen- 
tury between  530  and  430  B.  C.  there  were 
produced  fourteen  illustrious  men,  one  for 
every  4,300  of  the  free  born,  adult  male  popu- 
lation. In  the  two  centuries  from  500-300 
B.C.  this  small,  barren  country  with  an  area 


406  HEREDITY  AND  ENVIRONMENT 

and  total  population  about  equal  to  that  of  the 
present  State  of  Rhode  Island  but  with  less 
than  one-fifth  as  many  free  persons  produced 
at  least  twenty-five  illustrious  men.  Among 
statesmen  and  commanders  there  were  Milti- 
ades,  Themistocles,  Aristides,  Cimon,  Pericles, 
Phocion;  among  poets  ^Eschylus,  Euripides, 
Sophocles,  Aristophanes;  among  philosophers 
and  men  of  science  Socrates,  Plato,  Aristotle, 
Demetrius,  Theophrastus ;  among  architects 
and  artists  Ictinus,  Phidias,  Praxiteles,  Poly- 
gnotus;  among  historians  Thucydides  and 
Xenophon;  among  orators  ^Eschines,  Demos- 
thenes, Isocrates,  Lysias.  In  this  small  coun- 
try in  the  space  of  two  centuries  there 
appeared  such  a  galaxy  of  illustrious  men  as 
has  never  been  found  on  the  whole  earth  in 
any  two  centuries  since  that  time. 

These  illustrious  men  came  from  a  remark- 
able race  composed  of  individuals  drawn 
together  from  all  the  shores  of  the  Mediter- 
ranean by  a  process  of  unconscious  but  severe 
selection.  Athens  was  the  intellectual  and  so- 
cial capital  of  the  world  and  to  it  the  most 
ambitious  and  most  capable  men  were  irresist- 


CONTROL  OF  HEREDITY:     EUGENICS  407 

ibly  drawn.  It  was  good  immigration  as  well 
as  good  heredity  that  made  Athens  famous. 
Galton  concludes  that  the  average  ability  of 
the  Athenian  race  of  that  period  was,  on  the 
lowest  possible  estimate,  as  much  greater  than 
that  of  the  English  race  of  the  present  day  as 
the  latter  is  above  that  of  the  African  negro. 

But  this  marvellously  gifted  race  declined, 
as  all  such  races  have  in  time  declined : 

Social  morality  grew  exceedingly  lax,  marriage  be- 
came unfashionable  and  was  avoided,  many  of  the 
more  ambitious  and  accomplished  women  were 
avowed  courtesans  and  consequently  infertile,  and 
the  mothers  of  the  incoming  population  were  of  a 
heterogeneous  class.  ...  It  can  be  therefore  no  sur- 
prise to  us,  though  it  has  been  a  severe  misfortune 
to  humanity,  that  the  high  Athenian  breed  decayed 
and  disappeared,  for  if  it  had  maintained  its  excel- 
lence and  had  multiplied  and  spread  over  large 
countries,  displacing  inferior  populations  (which  it 
well  might  have  done,  for  it  was  naturally  very  pro- 
lific), it  would  assuredly  have  accomplished  results 
advantageous  to  human  civilization,  to  a  degree  that 
transcends  our  powers  of  imagination.  (Galton, 
"Hereditary  Genius,"  page  331.) 

3.  Why  the  Race  has  not  Improved. — If 
the  race  has  made  no  progress  in  hereditary 


408  HEREDITY  AND  ENVIRONMENT 

characteristics  since  the  time  of  the  Greeks  the 
cause  is  not  far  to  seek.  There  have  been 
gifted  races  and  gifted  families  of  men,  doubt- 
less many  notable  human  mutations  have  oc- 
curred, but  most  of  these  have  been  diluted, 
squandered,  lost.  There  has  been  persistent 
violation  of  all  principles  of  good  breeding 
among  men.  For  example,  there  has  been  for 
ages  a  futile  reliance  upon  good  environment 
to  improve  heredity.  Men  do  not  so  improve 
the  races  of  animals  and  plants,  and  thousands 
of  years  of  human  history  show  that  this 
method  is  of  no  avail  in  improving  the  human 
breed. 

But  the  case  is  far  worse  than  this; 
such  efforts  though  futile  are  at  least  well  in- 
tentioned,  but  on  the  part  of  most  men  and 
governments  there  has  been  complete  disre- 
gard of  the  entire  question  of  the  improvement 
of  the  human  stock.  Natural  selection  which 
has  through  countless  ages  eliminated  the 
worst  and  conserved  the  best  and  thus  has  led 
on  the  whole  to  the  survival  of  the  fit  is  so  far 
as  possible  nullified  by  civilized  man;  the 
worst  are  preserved  along  with  the  best  and 


CONTROL  OF   HEREDITY:     EUGENICS  409 

all  are  given  the  same  chance  of  reproduction. 
The  mistake  has  been  not  in  nullifying  natural 
selection  by  preserving  the  weak  and  incompe- 
tent, for  civilized  men  could  not  well  do  other- 
wise, but  in  failing  to  substitute  intelligent 
artificial  selection  for  natural  selection  in  the 
propagation  of  the  race.  Instead  of  this  there 
has  been  perpetuation  of  the  worst  lines 
through  sentimental  regard  for  personal 
rights,  even  when  opposed  to  the  welfare  of 
society ;  and  both  church  and  state  have  cheer- 
fully given  consent  and  blessing  to  the  mar- 
riage and  propagation  of  idiots  and  of  diseased, 
defective,  insane  and  vicious  persons.  Fi- 
nally there  has  been  extinction  of  the  world's 
most  gifted  lines  by  enforced  celibacy  in  many 
religious  orders  and  societies  of  scholars;  by 
almost  continuous  wars  which  have  taken  the 
very  best  blood  that  was  left  outside  of  the 
monastic  orders;  by  luxury  and  voluntary 
sterility;  by  vice,  disease  and  consequent 
infertility. 

Is  it  any  wonder  that  the  inheritance  of  the 
human  race  has  not  improved  within  historic 
times?  Is  it  not  rather  an  evidence  of  the 


410  HEREDITY  AND  ENVIRONMENT 

broadcast  distribution  of  good  and  wholesome 
qualities  in  the  race  that  in  spite  of  such 
serious  violations  of  the  principles  of  good 
breeding  mankind  remains  as  good  as  we  find 
it  to-day? 

III.  EUGENICS 

If  a  superior  power  should  deal  with  man  as 
man  deals  with  domestic  animals  no  doubt 
great  improvement  could  be  effected  in  the 
human  breed.  Society  is  in  some  respects 
such  a  power  and  can  do  what  the  individual, 
because  of  self-interest,  short  life  or  lack  of 
ability,  cannot  accomplish.  In  matters  of  pub- 
lic health,  comfort  and  security  of  life  and 
property  society  is  superior  in  power  to  the 
individual;  in  matters  of  the  perpetuation  of 
the  race  the  individual  is  still  supreme.  In 
animal  societies  the  race,  the  breed,  is  to  the 
swift  and  strong  and  fit,  and  the  same  was 
probably  true  of  primitive  men.  But  it  is  im- 
possible to  return  to  the  conditions  of  primi- 
tive society  in  this  respect,  and  the  social  body 
itself  must  in  some  way  control  the  breeding 
of  men. 


CONTROL  OF  HEREDITY:     EUGENICS          411 

There  are  millions  of  men  in  civilized 
countries  whose  mental  equipment  places  them 
on  a  plane  with  barbarians  or  savages,  and 
they  have  on  the  average  more  offspring  than 
their  civilized  contemporaries.  There  are  mil- 
lions of  others  who  are  so  seriously  defective 
in  body  or  mind,  owing  to  hereditary  causes, 
that  they  can  never  take  care  of  themselves  and 
must  always  be  a  charge  upon  the  state,  and 
yet  in  many  civilized  countries  they  are  per- 
mitted to  perpetuate  their  kind  and  produce 
an  ever-increasing  supply  of  mental  and 
moral  defectives,  whose  maintainance  must 
seriously  interfere  with  the  proper  education 
and  development  of  the  normal  population 
and  whose  unrestrained  existence  constantly 
threatens  to  pollute  purer  streams  of  heredity. 
The  practice  of  society  regarding  marriage 
and  reproduction  up  to  the  present  has  been 
to  allow  all  sorts,  good,  bad  and  indifferent,  to 
propagate  with  the  belief  that  good  environ- 
ment and  training  may  make  up  for  deficien- 
cies of  birth.  But  very  recently  the  conviction 
has  been  growing  that  good  environment  is  far 
less  important  than  good  heredity  and  that  in 


412  HEREDITY  AND  ENVIRONMENT 

some  way  society  must  influence  the  race  of 
men  at  its  source.  This  is  the  doctrine  of 
eugenics,  which  Galton  defines  as  follows: 

The  science  of  improving  stock,  which  is  by  no 
means  confined  to  questions  of  judicious  mating  but 
which,  especially  in  the  case  of  man,  takes  cog- 
nizance of  all  influences  that  tend  in  however  remote 
a  degree  to  give  to  the  more  suitable  races  or  strains 
of  blood  a  better  chance  of  prevailing  speedily  over 
the  less  suitable  than  they  otherwise  would  have  had. 
("Inquiries  into  Human  Faculty.") 

1.  Possible  and  Impossible  Ideals. — What 
the  future  evolution  of  the  human  race  may 
lead  to  is  an  interesting  speculation,  but  it  is 
and  can  be  only  a  speculation.  There  is  no 
present  evidence  that  there  will  ever  be  a 
higher  animal  than  man  on  the  earth,  and  the 
only  evidence  that  there  may  be  a  higher  spe- 
cies than  Homo  sapiens  is  to  be  found  in  the 
fact  that  there  have  been  lower  species  of  men 
in  the  past  and  that  evolution  has  been  on  the 
whole  progressive.  The  idea  that  by  the  aid 
of  that  infant  industry  eugenics  a  new  race  of 
supermen  is  shortly  to  be  produced  is  an  iri- 
descent dream,  and  the  fantastic  demand  of 
some  enthusiasts  for  changes  in  racial  fashions 


CONTROL  OF  HEREDITY:     EUGENICS          413 

has  served  to  bring' this  whole  subject  of  eu- 
genics into  disrepute  among  thoughtful  men. 

To  a  considerable  extent  the  ideals  regard- 
ing individuals  and  society  have  differed  in 
different  ages  and  races  in  the  past,  but  with 
the  closer  communications  which  have  been  es- 
tablished between  all  parts  of  the  earth  in 
modern  times  there  has  developed  a  greater 
uniformity  of  ideal.  In  a  complex  society  all 
types  of  service  are  needed  and  many  differ- 
ent types  of  individuals  are  socially  useful.  If 
the  social  good  were  the  supreme  end,  as  it  is 
in  a  colony  of  ants  or  bees,  the  greatest  dif- 
ferentiation of  individuals  for  particular  kinds 
of  service  would  be  desirable.  There  should 
be  a  hereditary  class  of  laborers,  of  business 
men,  of  scholars,  of  artists,  etc.,  and  for  the  im- 
provement of  each  class  there  should  be  in- 
breeding in  that  class.  Such  methods  are  now 
used  by  breeders  of  various  races  of  domestic 
animals  and  cultivated  plants  with  the  best  of 
results.  No  breeder  would  think  of  trying  to 
improve  draft  horses  by  crossing  with  race 
horses,  nor  of  improving  milk  cows  by  crossing 
with  beef  cattle.  In  other  countries  and  ages 


414  HEREDITY  AND  ENVIRONMENT 

the  development  of  hereditary  classes  and 
castes  in  human  society  has  been  tried,  and 
survivals  of  it  persist  to  this  day,  but  they  are 
only  vestigial  remnants  of  an  old  order  which 
is  everywhere  being  replaced  by  a  new  ideal  in 
which  the  good  of  the  individual  as  well  as  that 
of  society  is  the  end  desired. 

The  whole  development  of  modern  society 
is  in  the  direction  of  racial  solidarity  and  away 
from  hereditary  classes.  Government,  educa- 
tion and  religion;  socialism,  syndicalism,  an- 
archism all  reflect  the  movement  for  individual 
liberty,  fraternity  and  equality.  The  modern 
ideal  individual  is  not  the  highly  specialized 
unit  in  the  social  organism,  as  in  the  case  of 
social  insects,  but  rather  the  most  general  all- 
round  type  of  individual,  the  man  who  can 
when  conditions  demand  combine  within  him- 
self the  functions  of  the  laborer,  business  man, 
soldier  and  scholar.  For  such  a  generalized 
type  the  methods  of  inbreeding  or  close  breed- 
ing used  by  the  breeder  of  thoroughbreds  are 
wholly  inappropriate.  On  the  other  hand 
such  a  generalized  type  must  include  the  best 
qualities  of  many  types  and  races  and  Men- 


CONTROL  OF   HEREDITY:     EUGENICS  415 

delian  inheritance  shows  how  it  is  possible  to 
sort  out  the  best  qualities  from  the  worst. 

Nowadays  one  hears  a  lot  of  high  sounding 
talk  about  "human  thoroughbreds,"  which 
usually  means  that  those  who  use  this  phrase 
desire  to  see  certain  narrow  and  exclusive  so- 
cial classes  perpetuated  by  close  inbreeding;  it 
usually  has  no  reference  to  good  hereditary 
traits  wherever  found,  indeed  such  traits 
would  not  be  recognized  if  they  appeared  out- 
side of  "the  four  hundred."  Such  talk  prob- 
ably does  neither  harm  nor  good;  the  "social 
thoroughbreds"  are  so  few  in  number  and  so 
nearly  sterile  that  the  mass  of  the  population 
is  not  affected  by  these  exclusive  classes. 

Galton  advocated  the  segregation  and  inter- 
marriage of  the  most  highly  intellectual  mem- 
bers of  society,  such  as  the  prize  scholars  in  the 
colleges  and  universities;  but  if  the  human 
ideal  is  the  generalized  rather  than  the  spe- 
cialized type  it  would  be  better  if  the  prize 
scholars  married  the  prize  athletes.  A  race 
of  highly  specialized  scholars  or  athletes  is  not 
so  desirable  as  a  race  in  which  these  and  other 
excellencies  are  well  balanced.  From  this 


416  HEREDITY  AND  ENVIRONMENT 

point  of  view  the  person  who  is  voted  the  "best 
all  round  man  in  the  class"  is  nearer  the 
eugenical  ideal  than  the  prize  scholar. 

No  man  can  trace  his  lineage  back  through 
many  generations  without  realizing  that  it  in- 
cludes many  hereditary  lines  differing  greatly 
in  value.  The  significance  of  sexual  reproduc- 
tion lies  in  this  very  fact  that  it  brings  about 
the  commingling  of  distinct  lines  and  thereby 
makes  every  individual  different  from  every 
other  one.  The  entire  history  of  past  evolu- 
tion testifies  to  the  value  of  this  process,  al- 
though it  causes  the  gardener,  the  breeder,  the 
eugenicist  serious  trouble.  But  the  gardener 
can  propagate  his  choice  fruits  by  budding  and 
grafting,  the  breeder  can  for  a  time  preserve 
his  choice  stock  by  close  inbreeding,  but  the 
eugenicist  cannot  shut  out  the  influence  of  for- 
eign blood,  and  it  is  well  that  he  cannot  for 
if  he  could  do  so  the  progress  of  the  race 
would  probably  come  to  an  end. 

In  the  human  species  the  only  absolute  bar- 
rier to  the  intermingling  of  races  is  geographi- 
cal isolation.  Every  human  race  is  fertile  with 
every  other  one,  and  though  races  and  nations 


CONTROL  OF  HEREDITY:     EUGENICS  417 

and  social  groups  may  raise  artificial  barriers 
against  interbreeding  we  know  that  these  arti- 
ficial restraints  are  frequently  disregarded  and 
that  in  the  long  run  amalgamation  does  take 
place;  and  in  general  the  farther  amalgama- 
tion progresses  the  faster  it  goes.  In  Aus- 
tralia and  New  Zealand,  after  little  more  than 
a  century's  contact  with  white  races,  there  are 
about  as  many  "half  castes"  as  there  are  full 
blooded  aborigines.  In  the  United  States  one- 
quarter  of  all  persons  of  African  descent  con- 
tain more  or  less  white  blood;  there  are  about 
eight  million  full  blooded  negroes  and  two  mil- 
lion mulattoes,  and  during  the  past  twenty 
years  the  latter  have  increased  at  twice  the 
rate  of  the  former.  In  Jamaica,  where  there 
are  about  seven  hundred  thousand  blacks  and 
fifteen  thousand  whites,  there  are  about  fifty 
thousand  mulattoes.  A  similar  condition  pre- 
vails wherever  different  races  occupy  the  same 
country.  Even  the  Jews,  who  were  long  sup- 
posed to  be  a  peculiarly  separate  and  distinct 
people,  have  received  large  admixtures  of 
Gentile  blood  in  every  country  in  which  they 
have  lived. 


418  HEREDITY  AND  ENVIRONMENT 

Whether  we  want  it  or  not  hybridization  of 
human  races  is  going  on  and  will  increase. 
Partition  walls  between  classes  and  races  are 
being  broken  down;  complete  isolation  is  no 
longer  possible,  and  a  gradual  intermixture  of 
human  races  is  inevitable.  We  are  in  the  grip 
of  a  great  world  movement  and  we  cannot  re- 
verse the  current,  but  we  may  to  a  certain  ex- 
tent direct  that  current  into  the  more  desirable 
channels. 

There  is  a  popular  belief  that  hybrid  races 
are  always  inferior  to  pure  bred  ones,  but  this 
is  by  no  means  the  case.  Some  hybrids  are 
undoubtedly  inferior  to  either  of  the  parents 
but  on  the  other  hand  some  are  vastly  supe- 
rior; only  experience  can  determine  whether 
a  certain  cross  will  yield  inferior  or  superior 
types.  Society  may  well  attempt  to  prevent 
those  crosses  which  produce  inferior  stock 
while  encouraging  those  which  produce  su- 
perior types. 

It  is  this  fact  which  makes  the  problem  of 
immigration  so  serious.  In  general  immigra- 
tion is  regarded  merely  as  an  economic  and 
political  problem,  but  these  aspects  of  it  are 


CONTROL  OF   HEREDITY:     EUGENICS  419 

temporary  and  insignificant  as  compared  with 
its  biological  consequences.  In  welcoming  the 
immigrant  to  our  shores  we  not  only  share  with 
him  our  country  but  we  take  him  into  our 
families  and  give  to  him  our  children  or  our 
children's  children  in  marriage.  Whatever  the 
present  antipathies  may  be  to  such  racial  mix- 
tures we  may  rest  assured  that  in  a  few  hun- 
dred years  these  persons  of  foreign  race  and 
blood  will  be  incorporated  in  our  race  and  we 
in  theirs.  From  the  amalgamation  of  good 
races  good  results  may  be  expected ;  but  fusion 
with  inferior  races,  while  it  may  help  to  raise 
the  lower  race,  is  very  apt  to  pull  the  higher 
race  down.  How  insignificant  are  considera- 
tions of  cheap  labor  and  rapid  development  of 
natural  resources  when  compared  with  these 
biological  consequences ! 

2.  Negative  Eugenical  Measures. — Galton 
said  nothing  about  sterilization  or  elimination 
from  reproduction  of  less  valuable  lines  in  his 
"Inquiries  into  Human  Faculty"  which  was 
first  published  in  1883.  He  proposed  no  radi- 
cal policy  but  rather  one  which  he  thought 
would  be  practical  and  might  meet  with  general 


420  HEREDITY  AND  ENVIRONMENT 

favor.  He  suggested  a  social  policy  which 
would  delay  the  age  of  marriage  among  the 
weak  and  hasten  it  among  the  vigorous, 
whereas  present  social  agencies  act  in  the  op- 
posite direction.  He  showed  by  statistics  that 
on  the  average  marriage  at  the  age  of  22  would 
produce  at  the  end  of  one  century  four  times 
as  many  offspring  as  marriage  at  33  and  at 
the  end  of  two  centuries  ten  times  as  many. 
He  particularly  emphasized  the  great  harm 
which  would  be  done  by  an  application  of  the 
theory  of  Malthus  among  the  better  classes. 
For  the  prudent  to  put  off  marriage  and  to 
limit  offspring  while  the  imprudent  continue 
to  reproduce  at  the  present  rate  would  be  to 
give  the  world  to  the  imprudent  within  a  few 
centuries  at  most. 

His  suggestions,  which  were  at  first  received 
with  indifference  or  ridicule,  were  much  less 
radical  than  the  legal  requirements  in  many 
of  our  States  to-day.  Public  sentiment  has 
been  greatly  aroused  on  this  question;  the 
alarming  increase  in  the  number  of  defectives 
and  criminals  has  seemed  to  call  for  radical 
action  and  a  flood  of  hasty  but  well  intentioned 


CONTROL  OF   HEREDITY:     EUGENICS  421 

legislation  has  been  the  result.  We  may  con- 
fidently expect  that  in  a  very  short  time  the 
marriage  of  the  feeble-minded,  hopelessly 
epileptic  or  insane,  the  congenitally  blind, 
deaf  and  dumb,  and  those  suffering  from 
many  other  inherited  defects  which  unfit  them 
for  useful  citizenship  will  be  prohibited  by  law 
in  all  the  States.  Our  immigration  laws  al- 
ready exclude  such  aliens,  and  the  number  of 
persons  of  the  types  named  who  seek  legal 
consent  to  marry  is  not  large  so  that  it  need 
not  be  expected  that  such  laws  will  quickly 
improve  the  general  population.  If  in  addi- 
tion such  persons  are  either  segregated  or 
sterilized  the  danger  of  their  leaving  illegiti- 
mate offspring  will  be  removed;  such  pre- 
cautions have  been  taken  in  certain  of  our 
States  and  will  probably  become  general, 
though  at  present  few  of  the  laws  on  this  sub- 
ject are  strictly  enforced. 

The  study  of  heredity  shows  that  the  nor- 
mal brothers  and  sisters,  and  even  more  distant 
relatives,  of  affected  persons  may  carry  a  re- 
cessive defect  in  their  germ  plasm  and  may 
transmit  it  to  their  descendants  though  not 


422  HEREDITY  AND  ENVIRONMENT 

showing  it  themselves.  It  will  be  more  diffi- 
cult, perhaps  an  impossible  thing,  to  apply 
rigidly  the  principles  of  good  breeding  to  such 
persons  and  to  exclude  them  from  reproduc- 
tion ;  but  if  in  each  generation  those  persons  in 
whom  this  recessive  trait  appears  are  prevented 
from  leaving  offspring  the  number  of  persons 
affected  will  gradually  grow  less,  other  condi- 
tions being  equal. 

But  while  such  negative  eugenical  measures 
are  wholly  commendable  when  applied  to  such 
defects  as  those  named,  which  are  certainly  in- 
herited and  which  render  those  affected  unfit 
for  citizenship,  the  wholesale  sterilization  of 
all  sorts  of  criminals,  alcoholics  and  undesir- 
ables without  determining  whether  their  de- 
fects are  due  to  heredity  or  to  conditions  of 
development  would  be  like  burning  down  a 
house  to  get  rid  of  the  rats;  and  the  only 
justification  which  could  be  offered  for  the 
general  sterilization  of  the  inmates  of  all  pub- 
lic institutions,  which  is  urged  by  some  of  our 
modern  crusaders,  would  be  the  defense  which 
some  persons  make  for  war,  that  there  are 
too  many  people  anyway  and  anything  which 


CONTROL  OF  HEREDITY:     EUGENICS          423 

will  prevent  the  growth  of  population  is  to  be 
welcomed. 

Advocates  of  war  never  cease  to  point  out 
its  beneficial  effects  on  the  race, — how  it  makes 
men  strong,  courageous,  unselfish,  how  it 
makes  nations  great,  powerful,  progressive. 
There  is  no  doubt  that  war  like  any  other  great 
crisis  discovers  great  men  and  furnishes  op- 
portunities for  the  development  of  great  qual- 
ities that  might  otherwise  remain  undeveloped 
and  unknown.  But  there  is  also  no  doubt  that 
it  takes  the  very  best  blood  of  the  nations. 
Those  who  go  to  war  are  the  young,  the  strong, 
the  capable,  while  the  weak,  incompetent  and 
degenerate  are  left  behind  as  unfit  for  military 
service.  If  conditions  could  be  reversed  and 
the  bungled  and  botched,  the  feeble-minded 
and  insane,  the  degenerate  and  debauched 
could  be  put  in  the  fore  front  of  battle  some 
benefit  to  the  race  might  result,  but  no  increase 
of  national  greatness  can  compensate  for  the 
awful  waste  of  the  best  thing  which  any  nation 
possesses — its  best  blood. 

3.  Positive  Eugenical  Measures. — Positive 
eugenical  measures  are  much  more  difficult  to 


424  HEREDITY  AND  ENVIRONMENT 

apply  and  are  of  more  doubtful  value.  Of 
course  compulsory  measures  are  out  of  the 
question  and  encouragement  and  advice  alone 
are  feasible.  Giving  advice  regarding  matri- 
mony is  proverbially  a  hazardous  performance, 
and  it  is  not  much  safer  for  the  biologist  than 
for  others.  With  much  more  complete  knowl- 
edge regarding  human  inheritance  than  we 
now  possess  it  may  ultimately  be  possible  to 
give  such  advice  wisely,  especially  with  respect 
to  physical  characteristics  which  are  hereditar- 
ily simple  and  generally  of  minor  significance. 
But  where  the  character  is  an  extremely  com- 
plex one  as  in  intellectual  ability,  moral  recti- 
tude, judgment  and  poise,  which  are  the  chief 
characteristics  which  distinguish  the  great  man 
from  his  fellows,  it  will  probably  never  be  pos- 
sible to  predict  the  result  before  the  event. 
He  would  be  a  bold  prophet  who  would  un- 
dertake to  predict  the  type  of  personality  which 
might  be  expected  in  the  children  of  a  given 
union.  Some  very  unpromising  stocks  have 
brought  forth  wonderful  products.  Could 
anyone  have  predicted  Abraham  Lincoln  from 
a  study  of  his  ancestry?  Observe  I  say  "pre- 


CONTROL  OP  HEREDITY:     EUGENICS          425 

diet,"  and  not  "explain"  after  his  appearance. 
Can  anyone  now  predict  from  what  kind  of 
ancestral  combinations  the  great  scholars, 
statesmen,  men  of  affairs  of  the  next  gener- 
ation will  come?  The  time  may  come  when  it 
will  be  possible  to  predict  what  the  chances  are 
that  the  children  of  given  parents  will  inherit 
more  or  less  than  average  intellectual  capacity, 
but  since  germinal  potentiality  is  transformed 
into  intellectual  ability  only  as  the  result  of 
development  such  a  prediction  could  not  be 
extended  to  the  latter  unless  the  environment 
as  well  as  the  heredity  were  known. 

Mankind  is  such  a  mongrel  race,  good  and 
bad  qualities  are  so  mixed  in  us,  marriage  is 
such  a  lottery,  the  distribution  of  the  germinal 
units  to  the  different  germ  cells  and  the  union 
of  particular  germ  cells  in  fertilization  is  so 
much  a  matter  of  chance,  the  influence  of  even 
bad  hereditary  units  on  one  another  is  so  un- 
predictably  good  or  bad  as  is  shown  in  many 
hybrids,  even  the  minor  influences  of  environ- 
ment and  education  which  escape  attention  are 
so  potent  in  development,  that  the  chances 
were  infinity  to  one  against  any  one  of  us,  with 


426  HEREDITY  AND  ENVIRONMENT 

all  his  individual  characteristics,  ever  coming 
into  existence.  If  the  Greeks  or  Romans  had 
known  of  the  real  infinity  of  chances  through 
which  every  human  being  is  brought  to  the 
light  of  day  not  only  would  they  have  deified 
Chance  but  they  would  have  made  her  the 
mother  of  gods  and  men. 

But  granting  the  impossibility  of  predicting 
the  character  of  children  it  may  well  be  asked 
if  good  general  advice  may  not  be  given  re- 
garding the  choosing  of  a  mate.  Many  people 
have  thought  so,  and  if  all  that  has  been  said 
or  written  on  this  subject  were  to  be  gathered 
together  I  suppose  that  there  would  not  or 
should  not  be  room  for  it  in  all  the  libraries  of 
the  world.  It  is  generally  admitted  that  no 
lines  are  wholly  free  from  hereditary  defects 
and  the  question  has  often  been  asked  what 
the  eugenical  practice  should  be  in  such  cases. 
Of  course  people  with  really  serious  hereditary 
defects  should  not  have  children.  If  the  de- 
fects are  slight  Davenport  has  suggested  that 
they  may  be  either  disregarded  or  weakness 
in  any  character  may  be  mated  with  strength 
in  that  character.  That  people  with  only  slight 


CONTROL  OF  HEREDITY:     EUGENICS          427 

hereditary  defects  should  not  marry  at  all  is  a 
counsel  of  perfection. 

On  the  other  hand  it  would  be  a  dangerous 
rule  to  propose  that  persons  having  really 
serious  hereditary  defects  should  be  mated 
with  those  who  are  strong  in  those  characters 
on  the  ground  that  in  general  strength  in  a 
character  is  dominant  over  weakness.  It  has 
been  suggested  that  a  normal  man  who  marries 
a  feeble-minded  woman  would  have  only  nor- 
mal children,  since  both  genius  and  feeble- 
mindedness seem  to  be  recessive  when  mated 
with  mediocrity  or  normality.  But  in  all  such 
cases  the  weakness  is  not  neutralized  or  re- 
moved but  merely  concealed  in  the  offspring 
and  is  therefore  the  more  dangerous.  If  a  man 
chooses  to  marry  a  feeble-minded  woman  he  at 
least  does  so  with  his  eyes  open  and  he  need  not 
be  deceived.  But  the  normal  and  perhaps 
capable  children  of  such  a  union  carry  the  taint 
concealed  in  their  germ  plasm  and  if  they 
should  be  mated  with  other  normal  persons 
carrying  a  similar  taint  some  of  their  children 
would  be  feeble-minded,  and  thus  the  sins  of 
the  parents  in  mating  weakness  with  strength 


428  HEREDITY  AND  ENVIRONMENT 

would  be  visited  upon  the  children  of  the  nth 
generation.  Such  a  policy  of  concealing  weak- 
ness by  mating  it  with  strength  is  wholly  com- 
parable with  the  custom  once  prevalent  of 
concealing  cases  of  contagious  diseases,  and 
may  be  properly  characterized  as  the  "ostrich 
policy." 

After  all  in  the  choosing  of  mates  a  combi- 
nation of  instinct  and  intelligence  is  probably 
the  safest  guide.  Our  instincts,  built  up 
through  long  ages,  are  generally  adaptive  arid 
useful,  and  if  they  be  guided  by  reason  the  re- 
sult is  apt  to  be  better  than  if  either  instinct  or 
reason  act  alone.  More  need  not  be  said  on 
this  subject,  since  it  is  treated  ad  infinitum  in 
works  of  fiction  and  in  ladies'  journals. 

4.  Contributory  Eugenical  Measures. — In 
addition  to  the  negative  and  positive  eugenical 
measures  mentioned  many  conditions  may  be 
classed  as  contributary  to  eugenics.  One  of 
the  most  important  of  all  contributary  meas- 
ures is  the  general  education  of  the  people  re- 
garding heredity.  The  general  ignorance  on 
this  subject  is  profound  and  very  many  offen- 
ders against  the  principles  of  good  breeding 


CONTROL  OF   HEREDITY:    EUGENICS          429 

have  sinned  through  ignorance.  Any  general 
reform  must  rest  upon  enlightened  public 
opinion,  and  the  schools,  the  churches  and  the 
press  can  do  no  more  important  work  for  man- 
kind than  to  educate  the  people,  after  they 
have  been  educated  themselves,  on  this  im- 
portant matter.  . 

Society  too  may  cultivate  a  proper  pride  in 
good  inheritance.  Much  of  value  would  be  ac- 
complished if  the  silly  pride  in  ancestral 
wealth  or  position  or  environment  which 
touched  our  forebears  only  superficially  and 
never  entered  into  their  germ  plasm  or  the 
still  sillier  claims  of  long  descent,  in  which  we 
are  all  equal,  could  be  replaced  by  a  proper' 
pride  in  ancestral  heredity,  a  pride  in  those  in- 
herited qualities  of  body,  mind  and  character 
which  have  made  some  families  illustrious.  A 
proper  pride  in  heredity  would  do  much  to 
insure  the  perpetuation  of  the  line  and  to  pro- 
tect it  from  admixture  with  baser  blood. 

Among  other  contributory  measures  which 
serve  to  promote  good  breeding  among  men 
must  be  reckoned  coeducation,  as  well  as  other 
means  of  promoting  good  and  early  marriages. 


430  HEREDITY  AND  ENVIRONMENT 

The  president  of  a  large  coeducational  insti- 
tution once  said  that  if  marriages  were  made 
in  heaven  he  was  sure  that  the  Lord  had  a 
branch  office  in  his  university.  I  had  occasion 
a  year  ago  to  investigate  the  eugenical  record 
of  a  coeducational  institution,  which  is  not  un- 
known in  the  world  of  scholarship,  and  found 
that  about  33  per  cent,  of  the  recent  graduates 
had  married  fellow  students,  that  there  had 
been  no  divorces  and  that  there  were  many 
children.  There  is  no  doubt  that  coeducation 
promotes  good  and  early  marriages  and  that 
it  is  not  necessarily  inimical  to  good  scholar- 
ship even  though  it  violates  the  spirit  of 
'mediaeval  monasticism.  There  was  a  time  when 
it  was  supposed  that  a  scholar  must  live  the 
monkish  life  of  seclusion  and  contemplation, 
'but  the  monasteries  are  disappearing  the  world 
ever,  and  it  is  time  that  the  monastic  spirit 
should  go  out  of  the  colleges  and  universities. 

On  the  other  hand  the  colleges  exclusively 
for  women  appear  to  have  a  bad  influence  on 
the  marriage  rate  and  birth  rate  of  their  grad- 
uates. Johnson  has  shown  that  90  per  cent,  of 
all  the  women  of  the  United  States  marry  be- 


CONTROL  OF  HEREDITY:    EUGENICS          431 

fore  the  age  of  40,  but  that  among  college 
women  only  half  that  number  have  married  at 
the  same  age.  As  a  result  of  investigations  at 
one  of  the  leading  women's  colleges  he  finds 
that  the  marriage  and  birth  rate  of  the  most 
brilliant  students,  who  have  been  elected  mem- 
bers of  Phi  Beta  Kappa,  is  lowest  of  all.  Cat- 
tell  says  that  a  Harvard  graduate  has  on  the 
average  three-fourths  of  a  son,  a  Vassar  grad- 
uate one-half  of  a  daughter. 

At  present  early  and  fruitful  marriages 
among  able  and  ambitious  people  are  very  un- 
fashionable and  are  becoming  increasingly  im- 
practicable. If  society  has  any  regard  for  its 
own  welfare  all  this  must  be  changed.  As 
Galton  has  shown,  the  race  that  marries  at  22 
instead  of  33  will  possess  the  earth  in  two  or 
three  centuries. 

The  good  of  society  demands  that  we  re- 
verse our  methods  of  putting  a  premium  upon 
celibacy  among  our  most  gifted  and  ambitious 
young  men  and  women,  and  if  monastic  orders 
and  institutions  are  to  continue  they  should 
be  open  only  to  the  eugenicially  unfit. 

.5.  The  Declining  Birth  Mate  and  the  Death 


432  HEREDITY   AND   ENVIRONMENT 

of  Families. — Among  animals  and  plants  in  a 
state  of  nature  the  number  of  individuals  in 
each  species  remains  fairly  constant  from  year 
to  year;  that  is,  only  enough  young  are  born 
and  survive  to  take  the  places  of  mature  indi- 
viduals that  die.  But  when  a  species  is  placed 
in  new  and  favorable  conditions  it  may  for  a 
while  increase  at  an  amazing  rate  until  the 
pressure  of  population  becomes  sufficient  to 
reestablish  an  equilibrium  between  the  birth 
rate  and  the  death  rate.  Thus  when  the 
English  sparrow  was  introduced  into  the 
United  States  it  increased  at  a  phenomenal 
rate  for  a  number  of  years,  but  now  the  num- 
ber of  individuals  in  any  given  locality  remains 
about  the  same  from  year  to  year,  the  birth  rate 
merely  compensating  for  the  death  rate.  This 
equilibrium  is  brought  about  in  the  main  by 
increased  mortality,  especially  among  the 
young,  though  decreasing  fecundity  may  play 
a  minor  part. 

Essentially  the  same  principles  apply  to 
human  populations.  Up  to  two  or  three  cen- 
'turies  ago  the  populations  of  the  older  coun- 
tries of  the  world  were  practically  stationary. 


CONTROL  OF   HEREDITY:    EUGENICS  433 

Fecundity  was  relatively  high  but  the  death 
rate  was  also  very  high,  the  excess  of  popula- 
tion in  each  generation  being  carried  off  in 
large  numbers  by  war,  pestilence  and  famine. 
Then  owing  to  the  developments  of  science  and 
industry  and  to  the  opening  up  of  new  coun- 
tries a  period  of  remarkable  expansion  of  pop- 
ulation began.  The  population  of  Europe, 
which  was  about  175  millions  in  1800,  increased 
to  420  millions  in  1900,  and  this  in  spite  of  the 
fact  that  about  35  millions  migrated  from 
Europe  to  new  countries  during  this  period. 
This  great  increase  in  the  population  of  Eu- 
rope was  due  primarily  to  reduction  of  the 
death  rate  since  the  birth  rate  also  declined 
slightly  during  this  period,  while  in  the  newer 
countries  there  was  both  an  increase  in  the 
birth  rate  and  a  decrease  in  the  death  rate. 

It  is  perhaps  an  open  question  how  long  the 
advances  of  science  in  rendering  available  the 
natural  resources  of  the  earth  may  be  able  to 
keep  pace  with  increasing  population,  but  it  is 
evidently  impossible  for  this  great  increase  in 
the  population  of  the  world  to  go  on  in- 
definitely; sooner  or  later  it  must  come  to  an 


434  HEREDITY  AND  ENVIRONMENT 

end  and  the  population  again  become  station- 
ary. Already  the  birth  rate  is  decreasing  more 
rapidly  than  the  death  rate  in  all  the  western 
countries  of  Europe  and  this  movement  must 
ultimately  extend  to  all  parts  of  the  world  and 
lead  to  a  checking  of  the  great  increase  in 
population  which  has  characterized  the  last  two 
hundred  years.  This  approach  to  a  stationary 
population  is  both  a  normal  and  a  desirable 
thing,  for  no  one  could  wish  to  see  population 
increase  more  rapidly  than  the  supply  of  food 
or  other  necessaries  of  life ;  and  of  the  two  pos- 
sible methods  of  checking  population  few 
would  hesitate  to  choose  a  decreasing  birth  rate 
as  preferable  to  an  increasing  death  rate. 

It  is  not  therefore  the  declining  birth  rate  in 
the  general  population  that  should  cause 
alarm  but  rather  the  declining  birth  rate  in  the 
best  elements  of  a  population,  while  it  con- 
tinues to  increase  or  at  the  least  remains  sta- 
tionary among  the  poorer  elements,  and  there 
is  abundant  evidence  that  this  is  just  what  is 
taking  place.  The  descendants  of  the  Puri- 
tans and  the  Cavaliers  who  have  raised  the 
cry  for  "fewer  and  better  children"  are  already 


CONTROL  OF   HEREDITY:    EUGENICS  435 

disappearing  and  in  a  few  centuries  at  most 
will  have  given  place  to  more  fertile  races  of 
mankind.  Everybody  knows  that  the  old 
New  England  families  are  dying  out  and  that 
their  places  are  being  taken  by  recent  immi- 
grants. The  few  exceptions  are  merely  eddies 
in  the  current  that  is  bearing  them  to  doom. 
In  Massachusetts  the  birth  rate  of  the  foreign 
born  is  twice  that  of  the  native  population  while 
the  death  rate  is  about  the  same.  The  same 
is  true  of  the  older  families  in  many  parts  of 
the  world. 

Professor  Cattell  has  recently  made  a  sta- 
tistical study  of  the  families  of  917  Ameri- 
can men  of  science  and  he  finds  that  the 
average  size  of  family  of  the  parents  of  these 
men  was  4.66  children,  whereas  the  average 
size  of  family  of  these  men  is  2.22  children. 
In  one  generation  the  fertility  of  these  lines 
has  been  reduced  by  more  than  half.  The 
causes  of  this  decline  are  chiefly  voluntary 
being  assigned  to  health,  expense  and  other 
causes. 

But  the  causes  of  sterility  are  not  only  so- 
cial and  voluntary  ones,  which  could  be 


436  HEREDITY  AND  ENVIRONMENT 

changed  by  custom  and  public  opinion;  there 
are  also  involuntary  and  biological  causes  of 
a  deep-seated  nature.  Fahlenbeck  has  made 
a  study  of  433  noble  families  of  Sweden  which 
have  become  extinct  in  the  male  line,  and  he 
shows  that  the  last  male  died  unmarried  in 
45  per  cent,  of  these  families,  and  before  the 
age  of  21  in  39  per  cent.,  while  the  line  ended 
in  infertile  marriage  in  11  per  cent,  and  in 
daughters  only  in  5  per  cent. 

Broman  points  out  that  most  noble  families 
of  Europe  die  out  after  100  to  250  years  and 
generally  do  not  live  beyond  the  third  gener- 
ation. The  same  is  true  of  the  families  of 
great  scholars,  artists  and  statesmen.  Possi- 
bly one  cause  of  such  declining  fertility  may 
be  found  in  too  great  brain  activity,  but  there 
is  no  doubt  that  in  many  instances  it  is  due  to 
luxurious  living.  On  the  other  hand  bodily 
fatigue  and  simple  living  favor  fertility  in 
both  animals  and  men.  Wild  animals  brought 
into  captivity  where  they  have  comfortable 
quarters  and  an  unwonted  abundance  of  rich 
food  are  usually  infertile.  And  the  conditions 
of  life  of  the  upper  classes  of  society  are  al- 


CONTROL  OF  HEREDITY:    EUGENICS          437 

most  as  dangerous  for  fertility  as  is  captivity 
for  wild  animals.  It  is  evident  that  if  we  had 
fewer  luxuries  we  could  have,  and  could  afford 
to  have,  more  children. 

But  animals  in  captivity  may  gradually  be- 
come adapted  to  their  new  conditions  so  as  to 
become  fertile,  and  there  is  evidence  that  a 
slow  adaptation  through  several  generations 
to  conditions  of  high  civilization  is  possible. 
Some  royal  families  of  Europe  go  back  six  or 
eight  hundred  years,  and  in  general  if  a  family 
survives  the  new  conditions  of  affluence  arid 
luxury  for  more  than  three  generations  it  may 
become  more  or  less  adapted  to  the  new 
conditions. 

What  Bernard  Shaw  regards  as  the  greatest 
discovery  of  the  nineteenth  century,  viz.,  the 
means  of  artificially  limiting  the  size  of  fami- 
lies, may  prove  to  be  the  greatest  menace  to  the 
human  race.  If  it  were  applied  only  to  those 
who  should  not  have  children  or  to  those  who 
should  for  various  reasons  have  only  a  few  chil- 
dren it  would  be  a  blessing  to  mankind.  But 
applied  to  those  who  could  and  should  have 
many  children  it  is  no  gift  of  the  gods.  No 


438  HEREDITY  AND  ENVIRONMENT 

one  denies  that  the  chief  motive  for  limiting  the 
size  of  families  is  personal  comfort  and  pleas- 
ure rather  than  the  welfare  of  the  race.  The 
argument  that  people  should  have  no  more  chil- 
dren than  they  can  rear  in  comfort  or  luxury 
assumes  that  environment  is  more  important 
than  heredity,  which  is  contrary  to  all  the  bio- 
logical evidence.  In  the  breeding  of  horses  or 
cattle  or  men  heredity  is  more  potent  than 
environment ;  and  it  is  more  important  for  the 
welfare  of  the  race  that  children  with  good  in- 
heritance should  be  brought  into  the  world 
than  that  parents  should  live  easy  lives  and 
have  no  more  children  than  they  can  conven- 
iently rear  amid  all  the  comforts  of  a  luxury- 
loving  age. 

The  method  of  evolution  in  the  past  has  been 
the  production  of  enormous  numbers  of  indi- 
viduals and  the  elimination  of  the  least  fit.  The 
modern  method  of  improving  domestic  races  is 
to  select  for  reproduction  the  best  types  from 
large  numbers  of  individuals.  One  reason  why 
human  evolution  has  gone  on  so  slowly  is  to  be 
found  in  the  slow  breeding  of  men.  Nature 
has  provided  an  almost  infinite  wealth  and 


CONTROL  OF  HEREDITY:    EUGENICS          439 

variety  of  potential  personalities  in  human 
germ  cells  but  only  an  infinitesimal  number 
ever  come  to  development.  If  this  number  is 
still  further  reduced  by  artificial  means  the 
race  will  be  made  the  poorer  not  merely  in 
quantity  but  also  in  quality.  The  optimism  of 
those  who  believe  that  supermen  may  be  pro- 
duced by  artificially  limiting  the  number  of 
children  is  a  foolish  and  fatal  optimism. 

No  eugenical  reform  can  fail  to  take  account 
of  the  fact  that  the  decreasing  birth  rate  among 
intelligent  people  is  a  constant  menace  to  the 
race.  We  need  not  "fewer  and  better  chil- 
dren" but  more  children  of  the  better  sort  and 
fewer  of  the  worse  variety.  There  is  great 
enthusiasm  to-day  on  the  part  of  many  child- 
less reformers  for  negative  eugenical  meas- 
ures. The  race  is  to  be  regenerated  by 
sterilization.  But  unfortunately  this  reform 
begins  at  home  among  those  who  because  of 
good  hereditary  traits  should  not  be  sterile. 
Sterility  is  too  easily  acquired;  what  is  not  so 
easily  brought  about  is  the  fertility  of  the  bet- 
ter lines.  Galton  was  far  wiser  than  most  of 
his  followers  for  he  realized  the  necessity  of 


440  HEREDITY  AND  ENVIRONMENT 

increasing  the  families  of  the  better  types  as 
well  as  of  decreasing  those  of  the  worse. 

Finally  for  those  who  are  denied  the  privi- 
lege of  parenthood  and  upon  whom  sterility 
is  forced  by  whatever  circumstances  there  is  a 
lesson  of  value  among  the  social  insects.  The 
sterile  members  of  a  colony  of  ants  or  bees  are 
forever  denied  the  possibility  of  having  off- 
spring of  their  own,  but  they  become  foster 
mothers  to  the  offspring  of  the  queen.  They 
tenderly  nurse,  care  for  and  rear  the  young 
of  the  colony.  There  are  many  children  in  the 
world  who  need  foster  mothers  and  fathers; 
there  are  many  men  and  women  in  the  world, 
both  married  and  unmarried,  who  need  adop- 
ted children.  "Go  to  the  ant,  thou  sluggard; 
consider  her  ways  and  be  wise." 


CHAPTER  VI 
GENETICS  AND  ETHICS 


CHAPTER  VI 

GENETICS  AND  ETHICS1 
Modern  studies  of  development  are  pro- 
foundly changing  the  opinions  of  men  with  re- 
spect to  human  personality.  Observation  of 
the  relentless  laws  of  heredity,  of  the  inevitable 
influences  for  good  or  bad  of  environmental 
conditions  over  which  the  individual  has  no 
control,  undoubtedly  tends  to  produce  a  sense 
of  helplessness  and  hopelessness.  What  light 
is  thrown  upon  the  great  problems  of  freedom 
and  determinism,  of  responsibility  and  irre- 
sponsibility, of  duty  and  necessity  by  modern 
studies  of  development?  Such  questions  can- 
not be  dealt  with  quantitatively  and  experi- 
mentally, and  they  lie  outside  the  field  of  exact 
science,  but  they  are  involved  in  all  inquiries 
which  have  to  do  with  rational  and  social  be- 

1A  portion  of  this  lecture  was  given  as  the  presidential  ad- 
dress before  the  American  Society  of  Naturalists  in  January, 
1913,  and  was  published  in  Science  under  the'  title  "Heredity 
and  Responsibility." 

443 


444  HEREDITY  AND  ENVIRONMENT 

ings ;  they  lie  at  the  foundation  of  the  applica- 
tion of  science  to  human  welfare ;  they  occupy 
a  large  place  in  the  thought  and  conduct  of 
all  men. 

1.  THE  VOLUNTARISTIC  CONCEPTION  OF  NA- 
TURE AND  or  HUMAN  RESPONSIBILITY 

Primitive  men  regarded  their  own  activities 
and  all  phenomena  of  nature  as  the  expression 
of  will,  and  a  similar  view  has  been  maintained 
by  certain  philosophers  and  theologians  even 
in  modern  times.  Nature  was  regarded  as 
the  immediate  expression  of  a  vast  will  which 
creates,  rules,  builds  and  destroys  as  it  sees 
fit.  The  lightning  is  hurled  from  the  hand 
of  Jove,  the  sea  is  disturbed  by  angry  deities, 
the  winds  are  let  loose  or  stilled,  the  earth 
trembles,  the  hills  smoke,  the  sun  and  moon 
and  stars  travel  in  their  appointed  courses  as 
the  gods  will. 

In  this  primitive  view  of  nature  even  inani- 
mate objects  were  supposed  to  be  endowed 
with  wills  of  their  own,  and  many  modern  men 
are  sufficiently  primitive  to  kick  the  chair  over 
which  they  stumble,  or  to  swear  that  the  devil 


GENETICS  AND  ETHICS  445 

has  gotten  into  the  automobile.  Of  course  the 
actions  of  all  animate  things  were  held  to  be  the 
result  of  choice;  the  fly  that  dances  on  your 
head  or  gets  into  the  soup  is  doing  it  to  annoy 
you;  the  cats  that  yowl,  the  dogs  that  howl, 
the  maniacs  that  screech  are  possessed  of 
devils,  evil  wills,  and  should  be  punished.  All 
good  is  the  result  of  good  will,  all  evil  of  evil 
will.  Some  being,  some  volition,  is  responsi- 
ble for  everything  that  happens.  All  nature 
is  the  expression  of  big  or  little  wills,  of  good 
or  bad  wills,  and  the  good  should  be  rewarded 
and  the  bad  punished. 

This  conception  of  nature  finds  its  counter- 
part and  probably  its  origin  in  similar  views 
concerning  human  conduct  and  responsibility. 
According  to  this  belief  every  man  is  the  archi- 
tect of  his  own  character ;  the  will  is  absolutely 
free;  no  taint  of  heredity  or  necessity  rests  on 
the  mind  or  soul;  character  is  a  tabula  rasa, 
upon  which  the  self  writes  its  own  record  as 
it  chooses,  and  is  responsible  for  the  result. 
Conduct  whether  good  or  bad,  benevolent  or 
criminal,  rational  or  irrational  rests  upon  vol- 
untary choice,  and  for  such  choices  men  must 


446  HEREDITY  AND  ENVIRONMENT 

be  held  responsible.  To  a  great  extent  this 
view  of  freedom  and  responsibility  is  the  basis 
of  present  systems  of  government,  education, 
ethics  and  religion. 

II.  THE  MECHANISTIC  CONCEPTION  or  NA- 
TURE AND  OF  PERSONALITY 

As  contrasted  with  this  voluntaristic  view 
of  nature  and  of  man  consider  the  scientific 
conception  of  nature  as  a  vast  mechanism,  an 
endless  chain  of  causes  and  effects.  Science 
deals  with  "the  unfailing  order  of  immortal 
nature,"  with  the  universality  of  cause  and 
effect,  with  the  eternal  stability  and  inevita- 
bility of  natural  processes.  Natural  phe- 
nomena are  not  the  result  of  volitions  big  or 
little,  good  or  bad,  but  of  all  the  events  which 
have  gone  before.  To  the  man  of  science  na- 
ture is  not  the  mere  caprice  of  god  or  devil,  to 
be  lightly  altered  for  a  child's  whim;  nature 
is,  as  Bishop  Berkeley  said,  "stated,  fixed,  set- 
tled," eternal  process  moving  on,  the  same  yes- 
terday, to-day  and  forever. 

From  sands  to  stars,  from  the  immensity  of 
the  universe  to  the  minuteness  of  the  electron, 
in  living  things  no  less  than  in  lifeless  ones, 


GENETICS  AND  ETHICS  447 

science  recognizes  everywhere  the  inevitable  se- 
quence of  cause  and  effect,  the  universality  of 
natural  processes,  the  reign  of  natural  law. 
Man  also  is  a  part  of  nature,  a  part  of  the 
great  mechanism  of  the  universe,  and  all  that 
he  is  and  does  is  limited  and  prescribed  by  laws 
of  nature.  Every  human  being  comes  into  ex- 
istence by  a  process  of  development,  every  step 
of  which  is  determined  by  antecedent  causes. 

1.  The  Determinism  of  Heredity. — There 
can  be  no  doubt  that  the  main  characteristics 
of  every  living  thing  are  unalterably  fixed  by 
heredity.  Men  differ  from  horses  or  turnips 
because  of  their  inheritance.  Our  family  traits 
were  determined  by  the  hereditary  constitu- 
tions of  our  ancestors,  our  inherited  personal 
traits  by  the  hereditary  constitutions  of  our 
fathers  and  mothers.  By  the  shuffle  and 
deal  of  the  hereditary  factors  in  the  forma- 
tion of  the  germ  cells  and  by  the  chance  union 
of  two  of  these  cells  in  fertilization  our  heredi- 
tary natures  were  forever  sealed.  Our  ana- 
tomical, physiological,  psychological  possibili- 
ties were  predetermined  in  the  germ  cells  from 
which  we  came.  All  the  main  characteristics 


448  HEREDITY  AND  ENVIRONMENT 

of  our  personalities  were  born  with  us  and  can- 
not be  changed  except  within  relatively  nar- 
row limits.  "The  leopard  cannot  change  his 
spots  nor  the  Ethiopian  his  skin,"  and  "though 
thou  shouldst  bray  a  fool  in  a  mortar  with  a 
pestle  yet  will  not  his  foolishness  depart  from 
him."  Race,  sex,  mental  capacity  are  de- 
termined in  the  germ  cells,  perhaps  in  the 
chromosomes,  and  all  the  possibilities  of  our 
lives  were  there  fixed,  for  who  by  taking 
thought  can  add  one  chromosome,  or  even  one 
determiner  to  his  organization? 

The  thought  of  this  age  has  been  pro- 
foundly influenced  by  such  considerations. 
We  formerly  heard  that  "all  men  were  created 
free  and  equal";  we  now  learn  that  "all  men 
are  created  bound  and  unequal."  We  were 
once  taught  that  acts,  if  oft  repeated,  become 
habits,  and  that  habits  determine  character; 
hereditarians  of  the  stricter  sort  now  teach  that 
acts,  habits  and  character  were  foreordained 
from  the  foundation  of  the  family.  We  once 
thought  that  men  were  free  to  do  right  and 
wrong,  and  that  they  were  responsible  for 
their  deeds;  now  we  learn  that  our  reactions 


GENETICS  AND  ETHICS  449 

are  predetermined  by  heredity,  and  that  we 
can  no  more  control  them  than  we  can  control 
our  heart  beats.  For  ages  men  have  believed 
in  the  influence  of  example,  in  the  uplift  of 
high  ideals,  in  the  power  of  an  absorbing  pur- 
pose; for  ages  men  have  lived  and  died  for 
what  they  believed  to  be  duty  and  truth,  and 
have  received  the  homage  of  mankind ;  or  they 
have  lived  malevolent  and  criminal  lives  and 
have  been  despised  by  men  and  punished  by 
society.  But  if  our  reactions,  habits,  charac- 
ters are  predetermined  in  the  germ  plasm  such 
men  have  deserved  neither  praise  nor  blame. 
If  personality  is  determined  by  heredity  alone 
all  teaching,  preaching,  government  is  useless; 
freedom,  responsibility,  duty  are  delusions; 
whether  men  are  useful  or  useless  members  of 
society  depends  upon  their  inheritance,  and 
the  only  hope  for  the  race  is  in  eugenics — 
always  supposing  that  enough  freedom  is  left 
to  men  or  to  society  to  control  the  important 
function  of  choosing  a  mate. 

Already  a  few  enthusiastic  persons  have 
begun  to  apply  these  doctrines  to  practical  af- 
fairs. We  are  told  that  children  should  never 


450  HEREDITY  AND  ENVIRONMENT 

be  admonished  or  punished,  for  they  do  only 
what  their  natures  lead  them  to  do ;  the  nature 
of  the  child  must  be  respected  and  must  be 
allowed  to  manifest  itself  in  its  own  way.  Ly- 
ing and  stealing  will  cure  themselves  like  the 
mumps,  or  they  will  remain  incurable,  in  which 
case  the  germ  plasm  is  to  blame  and  nothing 
could  have  been  done  anyway.  Laziness  is  due 
to  inheritance  or  to  hookworms ;  the  latter  kind 
may  be  cured,  but  not  the  former.  Thriftless- 
ness,  alcoholism  and  uncleanness  run  in  fami- 
lies and  can  be  cured  only  by  extermination. 
Men  who  prey  upon  society  were  born  with 
wolfish  instincts,  and  cannot  help  but  eat  the 
lambs.  Villains,  lawbreakers,  murderers 
should  be  pitied  but  not  punished;  if  blame 
attaches  to  their  deeds  it  falls  upon  the  mar- 
riage bureau  and  the  parents.  The  world 
needs  hospitals  and  sanatoria  and  sterilization 
institutes  for  the  criminal  and  the  vicious,  but 
not  courts  and  prisons,  and  all  punishments 
should  be  visited  only  upon  the  parents  to  the 
third  and  fourth  generations. 

Do  our  studies  of  heredity  lead  us  to  any 
such  radical  conclusions?    If  they  do  we  must 


GENETICS  AND  ETHICS  451 

accept  them  like  brave  men.  "Truth  is  truth 
if  it  sears  our  eyeballs."  But  when  theories 
lead  to  such  revolutionary  results  it  behooves 
us  to  examine  carefully  those  theories  to  see  if 
there  is  not  somewhere  a  fundamental  flaw  in 
them. 

One  of  the  most  difficult  things  in  the  world 
is  to  recognize  a  great  truth,  to  feel  its  signi- 
ficance and  yet  not  be  carried  away  by  it. 
Great  scientific  errors  are  frequently  due  not 
so  much  to  faulty  observations  as  to  sweeping 
conclusions.  In  biology  the  search  for  univer- 
sal laws  is  a  peculiarly  dangerous  pursuit.  In 
philosophy  great  errors  are  often  due  not  so 
much  to  false  premises  as  to  supposed  logical 
necessities.  As  a  test  of  truth  logic  is  inferior 
to  experience ;  its  faults  are  not  so  much  in  its 
methods  as  in  its  premises  and  applications. 
For  this  reason  a  logical  chain  has  led  many 
a  man  into  the  bondage  of  error.  Truth  is  not 
usually  found  in  extremes,  in  "carrying  out  a 
process  to  its  logical  conclusions,"  but  rather 
in  some  middle  course  which  is  less  striking 
but  more  judicious. 

Having  observed  that  the  main  characteris- 


452  HEREDITY  AND  ENVIRONMENT 

tics  of  our  minds  as  well  as  of  our  bodies  are 
inherited,  it  is  easy  and  natural  to  go  further 
and  to  conclude  not  only  that  all  the  possibili- 
ties of  our  lives  are  marked  out  in  the  germ 
but  that  all  that  will  actually  develop  from  the 
germ  is  there  determined  and  cannot  be  al- 
tered. There  are  many  similarities  between 
such  an  extreme  view  and  the  old  doctrine  of 
preformation,  and  it  contains  a  like  absurdity. 
It  practically  denies  development  altogether. 
If  the  germ  is  a  closed  system  and  receives 
nothing  from  without,  and  if  adult  character- 
istics are  predetermined  in  the  germ,  they 
are  as  irrevocably  fixed  as  if  they  were 
predelineated. 

At  the  opposite  extreme  is  the  old  voluntar- 
istic  view  of  absolute  freedom  and  absolute 
responsibility.  This  view,  like  the  old  epigene- 
sis,  virtually  postulates  a  new  creation  for  each 
individual.  As  far  as  the  mind  and  soul  are 
concerned  there  is  no  hereditary  continuity 
with  past  generations  and  none  with  future 
ones.  But  while  such  a  view  may  be  logically 
complete  and  theologically  satisfying,  it  is  not 
scientific,  for  it  also  contradicts  the  evidence. 


GENETICS  AND  ETHICS  453 

The  truth  jheruseems  io_ Jie_ jsojnewliere. J>e- 
tweenjthesejtwajextremes..  Our  personalities 
were  not  absolutely  predetermined  in  the  germ 
cells  from  which  we  came,  and  yet  they  have 
arisen  from  those  germ  cells  and  have  been 
conditioned  by  them.  When  it  is  said  that  any 
characteristic  is  predetermined  in  the  germ 
cell,  what  does  this  mean?  What  but  that  the 
development  of  that  characteristic  is  made  pos- 
sible? Adult  characteristics  are  potential  and 
not  actual  in  the  germ,  and  their  actual  ap- 
pearance depends  upon  many  complicated  re- 
actions of  the  germinal  units  with  one  another 
and  with  the  environment.  In  short,  our  ac- 
tual personalities  are  not  predetermined  in  the 
germ  cells,  but  our  possible  personalities  are. 

2.  The  Determinism  of  Environment. — 
This  determinism  of  heredity  is  matched  by  a 
corresponding  determinism  of  environment. 
Life  is  possible  only  within  rather  narrow 
limits  of  physical  and  chemical  conditions  and 
in  the  main  these  limits  are  fixed  by  the  con- 
stitution of  nature.  But  apart  from  these  an- 
tecedent conditions  of  life  in  general  there  are 
many  minor  conditions  of  environment  which 


454  HEREDITY  AND  ENVIRONMENT 

exercise  a  profound  influence  upon  organisms, 
especially  in  the  course  of  their  development. 
Very  slight  changes  in  food,  temperature, 
moisture  and  atmospheric  conditions  may  pro- 
duce great  changes  in  the  developing  organ- 
ism, and  these  conditions  are  for  the  most 
part  entirely  beyond  the  control  of  the  indi- 
vidual affected. 

In  all  organisms  the  potentialities  of  de- 
velopment are  much  greater  than  the  actuali- 
ties. In  many  animals  a  small  part  of  the 
body  is  capable,  when  separated  from  the  re- 
mainder, of  producing  a  whole  body,  though 
this  potency  would  never  have  become  an  ac- 
tuality except  under  the  stimulus  of  separa- 
tion. In  like  manner  a  part  of  an  egg  may, 
when  separated  from  the  remainder,  give  rise 
to  an  entire  animal.  By  modifying  the  con- 
ditions of  development  animals  may  be  pro- 
duced which  have  one  eye,  many  eyes  or  no 
eyes;  animals  in  which  the  body  is  turned  in- 
side out,  or  side  for  side;  animals  in  which  all 
sorts  of  dislocation  of  organs  have  taken  place ; 
and  the  earlier  the  environmental  forces  act  the 
more  profound  are  the  modifications. 


GENETICS  AND  ETHICS  455 

But  leaving  out  of  account  all  forms  which 
are  so  monstrous  that  they  are  incapable  of 
reaching  maturity  we  find  that  there  are  left 
many  variations  in  the  size  and  vigor  of  the 
body  as  a  whole,  as  well  as  of  its  parts ;  many 
variations  in  the  more  or  less  perfect  correla- 
tion of  these  parts  with  one  another,  which 
were  determined  by  the  conditions  of  develop- 
ment rather  than  by  heredity.  In  a  given  germ 
cell  there  is  the  potency  of  any  kind  of  organ- 
ism that  could  develop  from  that  cell  under 
any  kind  of  conditions.  The  potencies  of  de- 
velopment are  much  greater  than  the  actuali- 
ties. Anything  which  could  possibly  appear 
in  the  course  of  development  is  potential  in 
heredity  and  under  given  conditions  of  en- 
vironment is  predetermined.  Since  the  en- 
vironment cannot  be  all  things  at  once  many 
hereditary  possibilities  must  remain  latent  or 
undeveloped.  Consequently  the  results  of  de- 
velopment are  not  determined  by  heredity 
alone  but  also  by  extrinsic  causes.  Things 
cannot  be  predetermined  in  heredity  which  are 
not  also  predetermined  in  environment. 

Of  all  animals  I  suppose  that  man  enjoys 


456  HEREDITY  AND  ENVIRONMENT 

the  most  extensive  and  the  most  varied  en- 
vironment, and  its  effect  upon  his  personality 
is  correspondingly  great.  Of  all  animals  man 
has  the  longest  period  of  immaturity  and  it  is 
during  this  period  that  the  play  of  environ- 
mental stimuli  on  the  organism  is  effective  in 
modifying  development.  In  addition  to  the 
material  environment  he  lives  in  the  midst  of 
intellectual,  social  and  moral  stimuli  which  are 
potent  factors  in  his  development.  By  means 
of  his  power  to  look  before  and  after  he  lives 
in  the  future  and  past  as  well  as  in  the  pres- 
ent; through  tradition  and  history  he  becomes 
an  heir  of  all  the  ages.  The  modifying  influ- 
ences of  all  these  environmental  conditions  on 
personality  are  very  great.  Each  of  us  may 
say  with  Ulysses:  "I  am  a  part  of  all  that  I 
have  met."  So  great  is  the  power  of  environ- 
ment on  the  development  of  personality  that  it 
may  outweigh  inheritance ;  a  relatively  poor  in- 
heritance with  excellent  environmental  condi- 
tions often  produces  better  results  than  a  good 
inheritance  with  poor  conditions.  Of  course 
no  sort  of  environment  can  do  more  than  bring 
out  the  hereditary  possibilities,  but  on  the 


GENETICS  AND  ETHICS  457 

other  hand  those  possibilities  must  remain 
latent  and  undeveloped  unless  they  are  stimu- 
lated into  activity  by  the  environment. 

Functional  activity  or  use  is  one  of  the 
most  important  factors  of  development.  Func- 
tional activity  is  response  to  stimuli,  which 
may  be  external  or  internal  in  origin.  The 
entire  process  of  development  may  be  regarded 
as  an  almost  endless  series  of  such  responses 
on  the  part  of  the  organism,  whether  germ 
cell,  embryo  or  adult,  to  external  and  internal 
stimuli.  It  is  a  truism  that  use  strengthens  a 
part  and  disuse  weakens  it;  it  is  likewise  a 
truism  that  responses  which  are  oft  repeated 
become  more  rapid  and  more  perfect,  and  in 
this  way  habits  are  formed.  Practically  all 
education,  whether  of  man  or  of  lower  ani- 
mals, consists  in  habit  formation,  in  establish- 
ing constant  relations  between  certain  external 
or  internal  stimuli  and  certain  responses  of  the 
organism.  At  first  these  stimuli  are  largely  of 
external  origin ;  later  the  external  stimuli  may 
be  replaced  more  and  more  by  internal  ones; 
but  whatever  the  source  of  the  stimulus  the 
response  of  the  organism  to  these  stimuli  is 


458  HEREDITY  AND  ENVIRONMENT 

one  of  the  most  important  factors  of  develop- 
ment, whether  of  the  body  or  of  the  mind. 

The  influence  of  environment  upon  the 
minds  and  morals  of  men  is  especially  great. 
To  a  large  extent  our  habits,  words,  thoughts ; 
our  aspirations,  ideals,  satisfactions;  our  re- 
sponsibility, morality,  religion  are  the  results 
of  the  environment  and  education  of  our  early 
years.  It  cannot  be  doubted  that  if  we  had 
been  born  in  other  countries  or  ages  we  should 
have  been  different  from  our  present  selves  in 
many  important  respects ;  if  we  had  been  born 
and  reared  in  the  slums  of  great  cities  we 
should  have  been  other  than  we  are;  indeed  if 
the  little  illnesses,  accidents  and  contingencies 
of  our  lives  had  been  different  we  should  have 
been  different  in  our  bodies  and  minds,  as  iden- 
tical twins  come  to  differ  from  each  other  under 
such  circumstances.  The  conditions  of  early 
life  and  education  have  a  great  influence  in 
shaping  personality  and  are  almost  as  much 
beyond  the  control  of  the  individual  as  is 
heredity. 

If  personality  in  all  of  its  main  features  is 
fixed  by  heredity  and  environment  over  which 


GENETICS  AND  ETHICS  459 

the  individual  has  little  or  no  qontrol,  and  this 
is  certainly  true,  personality  is  as  inevitably  de- 
termined by  its  antecedents  as  is  any  other 
natural  phenomenon.  This  is,  I  believe,  a 
conclusion  from  which  there  is  no  escape. 
How  then  is  it  possible  to  believe  in  freedom 
and  responsibility?  Is  there  not  justification 
for  the  view  so  often  expressed  of  late  that 
man  is  never  free  and  that  responsibility  and 
duty  are  mere  delusions? 

III.  DETERMINISM  AND  RESPONSIBILITY 

Many  persons  who  have  thought  upon  these 
subjects  have  felt,  apparently,  that  there  was 
no  tenable  middle  ground  between  extreme 
voluntarism  and  extreme  mechanism ;  man  has 
been  regarded  as  a  "free  agent"  or  a  mere 
"automaton,"  absolutely  free  or  absolutely 
bound,  wholly  indeterminate  or  wholly  prede- 
termined. But  these  extreme  views  are  unreal, 
unscientific  and  unjustifiable,  for  they  contra- 
dict the  facts  of  experience.  We  have  the  as- 
surance of  experience  that  we  are  not  abso- 
lutely free  nor  absolutely  bound,  but  that  we 


460  HEREDITY  AND  ENVIRONMENT 

are  partly  free  and  partly  bound;  the  alterna- 
tives are  not  merely,  freedom  or  determinism, 
but  rather  freedom  and  determinism. 

1.  Determinism  not  Fatalism. — Whatever 
the  philosophical  meaning  of  "determinism" 
may  be,  all  that  is  meant  by  that  term  in  science 
and  in  actual  life  is  that  every  effect  is  the  re- 
sultant of  antecedent  causes  and  that  identical 
causes  yield  identical  results.  Determinism 
does  not  mean  predeterminism :  the  one  finds 
every  effect  to  be  due  to  a  long  chain  of  pre- 
ceding causes,  the  other  attributes  every  effect 
to  a  single  original  cause;  the  one  is  scientific 
naturalism,  the  other  is  fatalism. 

Applying  .this  to  personality  actual  experi- 
ence teaches  that  constant  conditions  of  he- 
redity and  environment  give  constant  results 
in  development  and  that  different  conditions 
give  different  results.  Undoubtedly  the  entire 
personality,  body  and  mind,  undergoes  de- 
velopment, and  modifications  of  either  heredity 
or  environment  modify  personality.  This  is 
scientific  determinism,  but  it  is  not  fatalism 
and  it  is  not  incompatible  with  a  certain 
amount  of  freedom  and  responsibility. 


GENETICS  AND  ETHICS  461 

2.  Control  of  Phenomena  and  of  Self. — 
Even  the  most  extreme  mechanists,  who  main- 
tain that  we  are  mere  automata  and  that  we 
could  never  do  otherwise  than  we  do,  admit  the 
possibility  of  a  certain  amount  of  control  over 
phenomena  outside  ourselves.  They  tell  us 
that  the  aim  of  science  is  not  merely  to  under- 
stand but  also  to  control  nature.  But  if  man 
may  to  a  limited  extent  control  physical,  chem- 
ical and  biological  processes  in  the  world 
around  him,  if  he  may  control  to  a  limited  ex- 
tent the  behavior  of  a  star-fish  or  dog  or  child, 
on  what  ground  is  it  possible  to  deny  a  similar 
control  of  his  own  behavior?  Does  it  not  come 
to  this  that  all  such  control  means  intelligent 
action,  or  rather  the  introduction  of  intelli- 
gence as  a  factor  in  the  chain  of  cause  and 
effect?  Before  the  appearance  of  intelligence, 
whether  in  ontogeny  or  in  phylogeny,  no  such 
control  of  phenomena  or  of  self  is  possible, 
but  when  intelligence  becomes  a  factor  in  be- 
havior a  limited  control  of  the  world  and  of 
the  self  is  made  possible. 

Of  course  man  has  no  control  over  events 
which  have  already  happened.  Our  heredity 
and  early  development  are  accomplished  facts 


462  HEREDITY  AND  ENVIRONMENT 

which  nothing  can  change.  Development  is 
not  a  reversible  process;  a  man  cannot  enter 
a  second  time  into  his  mother's  womb  and  be 
born  again.  Once  the  sex  cells  are  formed 
their  hereditary  nature  is  determined ;  once  the 
egg  is  fertilized  the  hereditary  possibilities  of 
the  new  individual  are  fixed ;  once  any  stage  of 
development  has  passed  that  page  in  the  book 
of  life  is  closed  and  sealed. 

And  yet  at  every  step  in  this  long  process  of 
development  there  were  one  or  more  alterna- 
tives which  might  have  been  taken  instead  of 
the  one  which  was  taken.  There  were  innu: 
merable  possible  alternatives  in  the  matings  of 
our  ancestors,  there  were  billions  of  possible 
alternatives  in  the  union  of  the  millions  of 
types  of  germ  cells  which  each  of  our  parents 
produced ;  at  every  step  in  the  development  of 
the  oosperm  from  which  each  of  us  came  there 
were  many  possible  alternative  stimuli  and  re- 
sponses. But  in  each  case  one  of  these  in- 
numerable alternatives  was  taken  and  the  oth- 
ers left.  In  every  instance  there  was  some 
cause  that  determined  which  alternative  was 
taken,  but  these  causes  are  so  local  and  indi- 


GENETICS  AND  ETHICS  463 

vidual  that  they  cannot  be  generalized;  one 
cause  works  in  one  instance,  another  in  an- 
other, and  so  we  say  that  chance  determines 
which  alternative  is  taken,  meaning  by  chance 
only  this  that  the  causes  involved  cannot  be 
generalized.  At  critical  stages  in  this  process 
of  development  the  alternatives  are  so  evenly 
balanced  that  minor  considerations,  which  we 
call  chance,  determine  which  path  shall  be 
taken;  but  there  are  no  backward  steps  in  de- 
velopment and  once  a  path  has  been  taken 
that  particular  crisis  or  turning  point  does 
not  occur  again. 

Thus  each  of  us  has  wandered  through  the 
maze  of  life,  chance  usually  determining  which 
path  shall  be  taken  of  the  many  which  heredity 
and  environment  offer,  until  he  has  come  to 
a  stage  where  associative  memory  makes  it  pos- 
sible to  profit  by  experience  and  where  intel- 
lect and  will  make  possible  intelligent  choice. 
With  the  growth  of  intellect  and  will  there 
comes  to  be  a  limited  degree  of  freedom  and 
responsibility,  and  with  increasing  complexity 
of  organization  the  number  of  alternative 
paths  is  greatly  increased.  The  possible  reac- 


464  HEREDITY  AND  ENVIRONMENT 

tions  of  germ  cells  are  relatively  few  and 
fixed,  the  possible  reactions  of  a  complex  ani- 
mal are  relatively  many  and  behavior  is  more 
plastic;  and  thus  this  very  complexity  and 
plasticity  allow  adaptations  to  the  minutest  al- 
terations of  environment. 

3.  Birth  and  Growth  of  Freedom. — In  ani- 
mals below  man  and  in  the  stages  of  human 
development  one  may  trace  the  birth  and 
growth  of  freedom.  Even  in  some  of  the 
simplest  organisms  one  can  observe  inhibitions 
of  responses  and  modifications  of  behavior 
which  seem  to  be  due  to  conflicting  stimuli  or 
to  changes  in  the  physiological  state.  In 
higher  organisms  such  inhibitions  or  modifica- 
tions proceed  particularly  from  internal 
stimuli,  which  in  turn  are. probably  conditioned 
by  hereditary  constitution  and  past  experience. 
The  factors  which  determine  behavior  are  not 
merely  the  present  stimulus  and  the  heredi- 
tary constitution,  but  also  the  experiences 
through  which  the  organism  has  passed  and 
the  habits  which  it  has  formed. 

A  moth  cannot  avoid  the  impulse  to  fly 
toward  the  light,  and  it  does  not  learn  by  ex- 


GENETICS  AND  ETHICS  465 

perience  to  avoid  the  flame.  Its  reactions  are 
relatively  fixed  and  machine-like.  Many  other 
animals  learn  by  experience  to  inhibit  respon- 
ses to  certain  stimuli;  a  tame  fish  or  frog  will 
take  food  from  your  hand,  but  if  it  is  repeat- 
edly frightened  when  it  attempts  to  take  food 
it  will  not  come  near  you  though  it  is  starv- 
ing,— it  inhibits  the  strong  impulse  of  a 
hungry  animal  to  take  food  by  the  counter 
impulse  of  unpleasant  memories  or  of  fear. 
Here  we  have  the  beginnings  of  what  we  call 
freedom,  the  immediate  response  to  a  stimulus 
is  suppressed,  internal  stimuli  are  balanced 
against  external  ones  and  final  action  is  de- 
termined largely  by  past  experience.  Owing 
to  his  vastly  greater  power  of  memory,  reflec- 
tion and  inhibition  man  is  much  freer  than 
any  other  animal.  Animals  which  learn  little 
from  experience  have  little  freedom  and  the 
more  they  learn  the  freer  they  become. 

In  both  ontogeny  and  phylogeny  there  has 
been  development  of  freedom.  The  reactions 
of  germ  cells  and  of  the  lowest  organisms  are 
relatively  fixed.  In  more  complex  organisms 
reactions  become  modifiable  through  conflict- 


466  HEREDITY  AND  ENVIRONMENT 

ing  stimuli,  intelligence,  inhibitions.  Freedom 
is  the  more  or  less  limited  capacity  of  the  high- 
est organisms  to  inhibit  instinctive  and  non- 
rational  acts  by  intellectual  and  rational 
stimuli  and  to  regulate  behavior  in  the  light  of 
past  experience.  Such  freedom  is  not  un- 
caused activity,  but  freedom  from  the  mechan- 
ical responses  to  external  or  instinctive  stimuli, 
through  the  intervention  of  internal  stimuli 
due  to  experience  and  intelligence.  To  the 
person  accustomed  to  think  of  will  and  choice 
as  absolutely  free  this  may  seem  to  be  a  sort  of 
freedom  so  limited  as  to  be  scarcely  worth  the 
having;  and  yet  "it  is  the  dawning  grace  of  a 
new  dispensation,"  the  beginnings  of  rational 
life,  social  obligations,  moral  responsibility. 

The  only  control  over  natural  phenomena 
which  is  possible  is  in  choosing  between  alter- 
natives which  are  offered ;  and  the  only  control 
which  one  who  has  reached  the  age  of  intelli- 
gence can  have  over  his  own  development  con- 
sists in  choosing  between  the  alternatives  which 
are  open  to  him.  He  may  not  choose  his  he- 
redity or  early  development  for  the  alternative 
paths  which  were  once  offered  here  have  long 


GENETICS  AND  ETHICS  467 

since  been  passed;  but  to  a  limited  extent  he 
may  choose  his  present  environment  and  train- 
ing, he  may  choose  a  path  which  leads  to  dis- 
cipline and  increased  powers  of  self-control  or 
the  reverse,  and  to  this  extent  only  is  he  respon- 
sible for  what  he  may  become. 

4.  Responsibility  and  Will. — All  organisms 
are  capable  of  responding  to  chemical  and 
physical  stimuli  but  in  addition  normal  men 
have  the  capacity  of  responding  to  stimuli  of 
a  higher  order.  By  responsibility  in  this 
higher  sense  I  understand  the  ability  on  the 
part  of  the  organism  to  respond  to  rational, 
social  and  ethical  stimuli  or  impulses  and  to 
inhibit  responses  to  stimuli  of  an  opposite  na- 
ture, and  the  corresponding  expectation  on 
the  part  of  others  that  the  individual  will  so 
respond.  The  psychical  stimuli  which  influ- 
ence our  behavior  are  not  merely  remembered 
experiences  but  the  words,  suggestions,  ad- 
monitions, ideas  which  come  to  us  from  others, 
as  well  as  the  almost  endless  permutations 
of  such  memories  and  suggestions  in  our 
thoughts.  The  social  and  ethical  stimuli  are 
not  merely  such  as  arise  from  love  of  reward 


468  HEREDITY  AND  ENVIRONMENT 

and  fear  of  punishment  or  the  desire  for 
praise  and  the  fear  of  blame  but  also  from 
the  deep  seated  social  instinct  to  do  good, 
which  may  reach  the  highest  levels  of  altruism 
and  self  sacrifice. 

The  higher  the  type  of  organization  the 
larger  is  the  range  of  stimuli  to  which  it  will 
respond  and  the  larger  the  number  and  kind 
of  responses  which  may  be  called  forth;  and 
at  the  same  time  the  larger  becomes  the  power 
of  inhibition  of  responses  whether  through  the 
balancing  of  one  stimulus  against  another  or 
from  whatever  cause.  Human  responsibility 
varies  with  the  complexity  of  the  stimuli  in- 
volved- as  well  as  with  the  capacity  of  indi- 
viduals to  respond  to  those  stimuli.  A  man 
might  be  quite  responsible  in  savage  society 
who  would  be  quite  irresponsible  in  civilized 
communities.  In  an  infant  there  is  no  capacity 
to  respond  to  rational,  social  or  ethical  stimuli 
but  with  increasing  capacity  in  this  respect 
comes  increasing  responsibility.  Mental  and 
ethical  imbeciles,  insane  and  mentally  defective 
persons  have  a  low  capacity  for  such  responses 
and  inhibitions  and  consequently  less  is  ex- 


GENETICS  AND  ETHICS  469 

pected  of  them.  There  are  in  different  men 
all  degrees  of  responsibility,  as  there  are  all 
degrees  of  capacity.  In  one  and  the  same  in- 
dividual responsibility  varies  at  different  times 
and  under  different  circumstances ;  it  rises  and 
falls,  like  the  tides,  in  every  life.  Varying  ca- 
pacity to  respond  to  rational,  social  and  ethical 
stimuli  and  to  inhibit  responses  of  an  opposite 
nature  depends  not  merely  upon  inheritance 
but  also  upon  training,  habits,  physiological 
states.  The  common  opinion  that  all  normal 
men  are  equally  responsible  is  not  correct;  in 
the  eyes  of  the  law  this  may  be  true,  because 
legal  obligations  are  so  far  below  the  capacities 
of  normal  men  that  all  may  be  held  equally  re- 
sponsible before  the  law,  though  in  reality 
their  responsibilities  are  as  varied  as  their  in- 
heritance or  their  training. 

Conversely  the  responsibility  of  society  to 
the  individual  is  universally  recognized.  Ir- 
responsible persons  must  be  cared  for  by  older 
or  wiser  persons  who  become  responsible  for 
them;  and  in  general  the  responsibility  rests 
upon  society  to  provide  as  favorable  environ- 
ment as  possible  for  all  its  members.  Ex- 


470  HEREDITY  AND  ENVIRONMENT 

perienced  persons  can  to  a  certain  extent 
choose  their  own  environment  and  thus  indi- 
rectly control  their  responses  and  habits  but 
young  children  are  almost  if  not  quite  as  in- 
capable of  choosing  their  environment  as  of 
choosing  their  heredity,  and  it  becomes  the 
duty  of  society  to  see  to  it  that  the  environ- 
mental stimuli  are  such  as  to  develop  rational, 
social  and  ethical  habits  rather  than  the 
reverse. 

We  need  not  think  of  the  will  as  a  deus  ex 
machina,  nor  even  as  "a  little  deity  encapsuled 
in  the  brain,"  but  rather  as  the  sum  of  all  those 
psychical  processes,  such  as  memory  and  rea- 
son, which  regulate  behavior.  In  this  sense  the 
will  is  as  free  as  the  mind,  and  no  freer.  In- 
deed the  will  is  the  mind  acting  as  internal 
stimulus,  inhibition,  regulation;  in  this  sense 
the  existence  and  power  of  will  is  no  more  to 
be  doubted  than  the  existence  of  those  other 
mental  conditions  which  we  call  intellect  or 
memory. 

vjust  as  intellect  or  memory  may  be  trained 
to  accomplish  results  which  would  have  been 
impossible  to  the  untrained  mind,  so  will  may 


GENETICS  AND  ETHICS  471 

be  trained  to  initiate,  inhibit  or  regulate  be- 
havior in  a  manner  quite  impossible  to  one 
who  has  not  had  this  training.  It  is  one  of 
the  most  serious  indictments  against  modern 
systems  of  education  that  they  devote  so  much 
attention  to  training  memory  and  intellect  and 
so  little  attention  to  the  training  of  will,  upon 
the  proper  development  of  which  so  much 
depends. 

5.  Our  Unused  Talents. — Will  is  indeed 
the  supreme  human  faculty,  the  whole  mind 
in  action,  the  internal  stimulus  which  may  call 
forth  all  the  capacities  and  powers.  And  yet 
the  will  does  not  directly  create  nor  even  dis- 
cover these  powers;  they  are  produced  by  the 
factors  of  development,  by  heredity,  environ- 
ment and  training;  and  they  are  usually  dis- 
covered by  accident  or  under  the  stress  of 
necessity.  How  often  have  we  surprised  our- 
selves by  doing  some  unusual  or  prodigious 
task!  What  we  have  once  done  we  feel  that 
we  can  do  again.  We  realize  more  or  less 
clearly,  depending  upon  our  experience,  that 
what  we  habitually  do  is  far  less  than  we  could 
do.  It  is  this  reserve,  upon  which  we  can 


472  HEREDITY  AND  ENVIRONMENT 

draw  on  special  occasions,  that  gives  us  the 
sense  of  freedom. 

In  his  inspiring  address  on  "The  Energies 
of  Men"  William  James  showed  that  we  have 
reservoirs  of  power  which  we  rarely  tap,  great 
energies  upon  which  we  seldom  draw,  and  that 
we  habitually  live  upon  a  level  which  is  far 
below  that  which  we  might  occupy.  Darwin 
held  the  opinion,  as  the  result  of  a  lifetime  of 
observation,  that  men  differ  less  in  capacity 
than  in  zeal  and  determination  to  utilize  the 
powers  which  they  have.  In  playful  comment 
on  the  variety  and  extent  of  his  own  life  work 
he  said  in  modest  and  homely  phrase,  "It's 
dogged  as  does  it."  It  may  be  objected  that 
the  zeal  and  determination  were  inherited,  but 
here  also  the  hereditary  possibilities  become 
actualities  only  as  the  result  of  use,  training, 
the  formation  of  habits. 

It  is  generally  admitted  that  no  constant 
distinction  can  be  recognized  between  the 
brain  of  a  philosopher  and  that  of  many  a 
peasant.  Neither  size  nor  weight  of  brain  nor 
complexity  of  convolutions  bears  any  constant 
relation  to  ignorance  or  intelligence,  though 


GENETICS  AND  ETHICS  473 

doubtless  an  "unlimited  microscopist"  could 
find  differences  between  the  trained  and  the 
untrained  brain.  The  brains  of  Beethoven, 
Gauss  and  Cuvier,  although  unusually  large, 
have  been  matched  in  size  and  visible  complex- 
ity by  the  brains  of  unknown  and  unlearned 
persons— persons  who  were  richly  endowed  by 
nature  but  who  had  never  learned  to  use  their 
talents.  In  all  men  the  capacity  for  intellec- 
tual development  is  probably  much  greater 
than  the  actuality.  The  parable  of  the  talents 
expresses  a  profound  biological  truth,  men 
differ  in  hereditary  endowments,  one  receives 
ten  talents  and  another  receives  but  one;  but 
the  used  talent  increases  many  fold,  the  un- 
used remains  unchanged  and  undeveloped. 
Happy  is  he  who  is  compelled  to  use  his  tal- 
ents; thrice  happy  he  who  has  learned  how 
to  compel  himself!  We  shall  not  live  to  see 
the  day  when  human  inheritance  is  greatly  im- 
proved, though  that  time  will  doubtless  come, 
but  in  the  meantime  we  may  console  ourselves 
by  the  thought  that  we  have  many  half -used 
talents,  many  latent  capacities,  and  although 
we  may  not  be  able  to  add  to  our  inheritance 


474  HEREDITY  AND  ENVIRONMENT 

new  territory  we  may  greatly  improve  that 
which  we  have. 

Jennings  has  pointed  out  as  one  of  the  great 
tragedies  of  life  the  almost  infinite  slaughter 
of  potential  personalities  in  the  form  of  germ 
cells  which  never  develop.  A  more  dreadful 
though  less  universal  tragedy  is  the  loss  of 
real  personalities  who  have  all  the  native  en- 
dowments of  genius  and  leadership  but  who 
for  lack  of  proper  environmental  stimuli  have 
remained  undeveloped  and  unknown;  the 
"mute,  inglorious  Miltons"  of  the  world;  the 
Cassars,  Napoleons,  Washingtons  who  might 
have  been;  the  Newtons,  Darwins,  Pasteurs 
who  were  ready  formed  by  nature  but  who 
never  discovered  themselves.  One  shudders 
to  think  how  narrowly  Newton  escaped  being 
an  unknown  farmer,  or  Faraday  an  obscure 
bookbinder,  or  Pasteur  a  provincial  tanner. 
In  the  history  of  the  world  there  must  have 
been  many  men  of  equal  native  endowments 
who  missed  the  slender  chance  which  came 
to  these.  We  form  the  habit  of  thinking  of 
great  men  as  having  appeared  only  at  long 
intervals,  and  yet  we  know  that  great  crises  al- 


GENETICS  AND  ETHICS  475 

ways  discover  great  men.  What  does  this 
mean  but  that  the  men  are  ready  formed  and 
that  it  requires  only  this  extra  stimulus  to  call 
them  forth?  To  most  of  us  heredity  has  been 
kind — kinder  than  we.  know.  The  possibilities 
within  us  are  great  but  they  rarely  come  to  full 
epiphany. 

What  is  needed  in  education  more  than  any- 
thing else  is  some  means  or  system  which  will 
train  the  powers  of  self  discovery  and  self 
control.  Easy  lives  and  so-called  "good  en- 
vironment" will  not  arouse  the  dormant 
powers.  It  usually  takes  the  stress  and  strain 
of  hard  necessity  to  make  us  acquainted  with 
our  hidden  selves,  to  rouse  the  sleeping  giant 
within  us.  How  often  is  it  said  that  the 
worthless  sons  of  worthy  parents  are  mys- 
teries; with  the  best  of  heredity  and  environ- 
ment they  amount  to  nothing,  whereas  the  sons 
of  poor  and  ignorant  farmers,  blacksmiths, 
tanners  and  backwoodsmen,  with  few  oppor- 
tunities and  with  many  hardships  and  disad- 
vantages, become  world  figures.  Probably  the 
inheritance  in  these  last  named  cases  was  no 
better  than  in  the  former,  but  the  environment 


476  HEREDITY  AND  ENVIRONMENT 

was  better.  "Good  environment"  usually 
means  easy,  pleasant,  refined  surroundings, 
"all  the  opportunities  that  money  can  buy," 
but  little  responsibility  and  none  of  that  self 
discipline  which  reveals  the  hidden  powers  and 
which  alone  should  be  counted  good  environ- 
ment. Many  schools  and  colleges  are  making 
the  same  mistake  as  the  fond  parents;  luxury, 
soft  living,  irresponsibility  are  not  only  al- 
lowed, but  are  encouraged  and  endowed — and 
by  such  means  it  is  hoped  to  bring  out  that  in 
men  which  can  only  be  born  in  travail. 

The  chief  educational  value  of  athletics  is 
found  in  this  that  it  teaches  self  control.  But 
in  great  athletic  contests  the  self  control  of  the 
spectators  is  usually  inversely  proportional  to 
that  of  the  players,  and  while  excess  of  stimuli 
}  may  lead  to  wholesome  and  beneficial  reactions 
i  in  the  players  it  frequently  leads  to  excess  of 
stimulants  and  to  other  injurious  reactions  in 
the  spectators.  But  college  athletics  has  this 
much  at  least  in  its  favor,  it  trains  men  who 
take  part  in  the  contests  to  do  their  best,  to 
subordinate  pleasure,  appetite,  the  desire  for 
a  good  time,  to  one  controlling  purpose ;  it 


I 


GENETICS  AND  ETHICS  477 

trains  them  to  attempt  what  may  often  seem 
to  them  impossible,  to  crash  into  the  hue 
though  it  may  seem  a  stone  wall,  to  get  out 
of  their  bodies  every  ounce  of  strength  and 
endurance  which  they  possess.  Such  training 
makes  men  acquainted  with  their  powers  and 
teaches  courage,  confidence  and  responsibility. 
If  only  we  could  make  young  persons  ac- 
quainted in  some  similar  way  with  their  hidden 
mental  and  moral  powers  what  a  race  of  men 
and  women  might  we  not  have  without  wait- 
ing for  that  uncertain  day  when  the  inheri- 
tance of  the  race  will  be  improved !  Whatever 
the  stimulus  required,  whether  pride  or  shame, 
fear  or  favor,  ambition  or  loyalty,  responsi- 
bility or  necessity,  education  should  utilize 
each  and  all  of  these  to  teach  men  self  knowl- 
edge and  self  control. 

But  it  will  be  said  that  self  control  depends 
upon  inheritance,  that  strong  wills  and  weak 
wills  are  such  because  of  heredity.  It  is  true 
that  one  man  may  be  born  with  a  potentiality 
for  self  control  which  another  man  lacks,  but 
in  all  men  this  potentiality  becomes  actuality 
only  through  development,  one  of  the  princi- 


478  HEREDITY  AND  ENVIRONMENT 

pal  factors  of  which  is  use  or  functional  ac- 
tivity. An  amazing  number  of  persons  have 
but  little  self  control.  Is  this  always  due  to 
defective  inheritance,  or  is  it  not  frequently 
the  result  of  bad  habits,  of  arrested  develop- 
ment? To  charge  defects  at  once  to  heredity 
removes  them  from  any  possible  control,  helps 
to  make  men  irresponsible,  excuses  them  for 
making  the  least  of  their  endowments.  To 
hold  that  everything  has  been  predetermined, 
that  nothing  is  self  determined,  that  all  our 
traits  and  acts  are  fixed  beyond  the  possibility 
of  change  is  an  enervating  philosophy  and  is 
not  good  science,  for  it  does  not  accord  with 
the  evidence.  It  is  amazing  that  men  whose 
daily  lives  contradict  this  paralyzing  philoso- 
phy still  hold  it,  as  it  were  in  some  water- 
tight compartment  of  the  brain,  while  in  all 
the  other  parts  of  their  being  their  acts  pro- 
claim that  they  believe  in  their  powers  of  self 
control:  they  set  themselves  hard  tasks,  they 
overcome  great  difficulties,  they  work  until  it 
hurts,  until  they  can  say  with  Johannes  Miil- 
ler,  Es  Tdebt  Blut  an  der  Arbeit,  and  yet  in 
the  philosophical  compartment  of  their  minds 


GENETICS  AND  ETHICS  479 

they  can  say  that  it  was  all  predetermined  in 
heredity  and  from  the  foundations  of  the 
world. 

Whether  all  the  phenomena  of  life  and  of 
mind  can  be  explained  on  the  basis  of  a  purely 
mechanistic  hypothesis  or  not,  that  hypothesis 
must  square  with  the  facts  and  not  the  facts 
with  the  hypothesis.  It  has  always  been  true 
of  those  who  "sat  apart  and  reasoned  high 
of  fate,  free  will,  foreknowledge  absolute"  that 
they  have  "found  no  end  in  wandering  mazes 
lost."  Whatever  the  way  out  of  these  mazes 
may  be, — whether  it.  be  found  in  the  varied  re- 
sponses of  an  organism  to  the  same  stimulus, 
to  the  introduction  of  memory,  intelligence 
and  reason  as  internal  stimuli,  or  to  some  form 
of  idealism  which  finds  necessity  not  in  nature 
but  in  the  spectator,  and  freedom  not  in  the 
spectator  but  in  the  agent, — it  is  true  for  those 
who  do  not  "sit  apart  and  reason  high,"  but 
who  deal  merely  with  evident  phenomena,  that 
the  way  out  of  these  mazes  is  not  to  be  found 
in  denying  the  reality  of  inhibition,  atten- 
tion and  control.  Because  we  can  find  no 
place  in  our  philosophy  and  logic  for  self  de~ 


480  HEREDITY  AND  ENVIRONMENT 

!  termination  shall  we  cease  to  be  scientists  and 
close  our  eyes  to  the  evidence?  The  first  duty 
of  science  is  to  appeal  to  fact  and  to  settle 
later  with  logic  and  philosophy.  Is  it  not  a 
fact  that  the  possibilities  of  our  inheritance 
depend  for  their  realization  upon  development, 
one  of  the  most  important  factors  of  which  is 
use,  functional  activity  in  response  to  stimuli? 
Is  it  not  a  fact  that  in  many  animals  behavior 
is  modifiable  and  that  impulses  may  be  in- 
hibited and  controlled?  Is  it  not  a  fact  that 
experience,  intelligence,  will  are  factors  in 
human  behavior  and  that  by  means  of  these 
men  are  often  able  to  choose  between  alterna- 
tives and  so  to  control  their  own  activities  as 
well  as  external  phenomena?  Is  it  not  a  fact 
that  our  capacities  are  very  much  greater  than 
our  habitual  demands  upon  them?  Is  it  not 
a  fact  that  belief  in  our  responsibility  ener- 
gizes our  lives  and  gives  vigor  to  our  mental 
and  moral  fiber?  Is  it  not  a  fact  that  shifting 
all  responsibility  from  men  to  their  heredity 
or  to  that  part  of  their  environment  which  is 
beyond  their  control  helps  to  make  them 
irresponsible  ? 


GENETICS  AND  ETHICS  481 

This  debilitating  philosophy  in  which  every- 
thing is  predetermined,  in  which  there  is  no 
possibility  of  change  or  control,  in  which  there 
is  hypertrophy  of  intellect  and  atrophy  of  will 
is  a  symptom  of  senility  whether  in  men  or 
nations.  We  need  to  return  to  the  joys  of  a 
childhood  age  in  which  men  believed  them- 
selves free  to  do,  to  think,  to  strive,  in  which 
life  was  full  of  high  endeavor  and  the  world 
was  crowded  with  great  emprise.  We  need  to 
think  of  the  possibilities  of  development  as 
well  as  of  the  limitations  of  heredity.  Chance, 
heredity,  environment  have  settled  many 
things  for  us ;  we  are  hedged  about  by  bounds 
which  we  cannot  pass,  but  those  bounds  are 
not  so  narrow  as  we  are  sometimes  taught  and 
within  them  we  have  a  considerable  degree  of 
freedom  and  responsibility. 

"That  which  we  are  we  are, 
One  equal  temper  of  heroic  hearts 
Made  weak  by  time  and  fate,  but  strong  in  will 
To  strive,  to  seek,  to  find,  and  not  to  yield." 


482  HEREDITY  AND  ENVIRONMENT 


IV.  THE  INDIVIDUAL,  AND  THE  RACE 
There  is  a  larger  freedom  and  a  greater 
responsibility  than  that  which  characterizes  the 
individual.  What  the  individual  cannot  do 
because  of  weakness,  ignorance,  self  interest, 
short  life,  society  can  accomplish  with  the 
strength,  wisdom,  and  interest  of  all,  and 
through  long  ages  of  time.  There  are  many 
grades  of  organization  from  the  bacterium  to 
the  vertebrate,  from  the  germ  cell  to  the  man. 
Society  is  the  last  and  highest  grade  of  organi- 
zation and  its  freedom  and  responsibility  are  to 
those  of  the  individual  very  much  as  the  free- 
dom and  responsibility  of  the  developed  man 
are  to  those  of  the  germ  cell  from  which  he 
came.  Out  of  the  correlations,  differentiations 
and  integrations  of  persons  has  grown  this 
higher  type  of  organization  which  we  call 
society. 

1.  The  Conflict  between  the  Freedom  of  the 
Individual  and  the  Good  of  Society. — The 
freedom,  power  and  responsibility  of  society 
are  founded  upon  limitations  of  individual  free- 


GENETICS  AND  ETHICS  483 

dom  for  the  good  of  the  race.  Among  social 
animals,  such  as  ants  and  bees,  there  is  so 
much  instinct  and  so  little  reason  and  freedom 
that  there  is  practically  no  conflict  between 
the  individual  and  the  race,  but  with  the  in- 
crease of  intelligence  and  freedom  among  men 
there  has  developed  an  increasing  conflict  be- 
tween the  individual  and  society.  So  far  as 
social  limitations  are  artificial,  selfish,  for  the 
good  of  a  few  rather  than  of  all,  this  conflict  of 
the  ages,  this  struggle  to  be  free  has  been  the 
crowning  glory  of  mankind.  The  struggle 
for  freedom  from  tyranny  in  thought  and 
speech,  in  religion,  government  and  industry, 
no  less  than  for  the  freedom  that  comes  by 
the  conquest  of  nature,  is  one  of  the  greatest 
achievements  of  the  human  race. 

But  social  restrictions  on  individual  freedom 
are  not  all  artificial  and  selfish.  Some  of  them 
are  absolutely  essential  not  only  to  the  welfare 
but  even  to  the  continued  existence  of  the  race* 
and  when  demands  for  individual  freedom  go 
to  the  extent  of  fighting  against  these  racial 
obligations  they  become  a  serious  menace  to 
mankind. 


484  HEREDITY  AND  ENVIRONMENT 

2.  Perpetuation  and  Improvement  of  the 
Race  the  highest  Ethical  Obligation. — Among 
all  organisms  the  race  or  species  is  of  para- 
mount importance.  Race  preservation,  not 
self  preservation,  is  the  first  law  of  nature. 
Among  all  organisms  the  perpetuation  and 
welfare  of  the  race  are  cared  for  by  the  strong- 
est instincts.  In  very  many  species  of  animals 
reproduction  means  the  death  of  the  individual. 
The  breeding  instinct  drives  every  male  bee, 
every  male  and  female  salmon,  to  its  certain 
death  in  order  that  the  race  may  be  perpetu- 
ated. Among  the  higher  organisms  the 
strongest  of  all  the  instincts  are  those  con- 
nected with  reproduction.  But  in  the  human 
species  intellect  and  freedom  come  in  to  inter- 
fere with  instinct.  The  reproductive  instincts 
are  not  merely  controlled  by  reason,  as  they 
should  be,  but  to  an  alarming  extent  they  are 
thwarted  and  perverted  among  intelligent 
people. 

The  struggle  to  be  free  is  part  of  a  great 
evolutionary  movement,  but  the  freedom  must 
be  a  sane  one  which  neither  injures  others  nor 
eliminates  posterity.  The  feminist  movement 


GENETICS  AND  ETHICS  485 

in  so  far  as  it  demands  greater  intellectual  and 
political  freedom  for  women  may  be  a  benefit 
to  the  race  but  in  so  far  as  it  demands  freedom 
from  marriage  and  reproduction  it  is  suicidal. 
The  cry  of  Rachel,  "Give  me  children  or  I 
die,"  has  been  turned  by  many  modern  women 
to,  "I'd  rather  die  than  have  children."  If 
the  demand  for  individual  freedom  blinds  men 
and  women  to  their  racial  obligations  the  in- 
evitable decadence  and  extinction  of  their  lines 
must  follow.  In  every  age  and  country  where 
demands  for  personal  freedom  have  been  most 
insistent  and  extreme,  where  men  and  espe- 
cially women  have  demanded  freedom  from  the 
burdens  of  bearing  and  rearing  children  as 
well  as  from  other  natural  social  obligations, 
the  end  has  been  degeneration  and  extinction. 
This  has  been  the  history  of  many  talented 
races  and  families  of  mankind.  The  decay  of 
the  most  gifted  races  of  the  ancient  world, 
especially  those  of  Greece  and  Rome,  was  not 
due  primarily  to  bad  heredity  nor  to  bad  ma- 
terial environment  but  rather  to  the  growth 
of  luxury  and  selfishness  and  unrestricted  free- 
dom; marriage  became  unfashionable,  immo- 


486  HEREDITY  AND  ENVIRONMENT 

rality  was  widespread,  and  then  came  sterility 
and  extinction  or  mixture  with  inferior  stock 
and  degeneracy.  And  then  the  barbarian,  the 
immigrant,  the  natural  man,  unspoiled  by  too 
much  freedom  and  true  to  his  instincts,  came 
in  to  take  the  place  of  the  more  gifted  race. 
Truly  "there  is  a  power  not  ourselves  that 
makes  for  righteousness." 

In  these  days  when  we  talk  of  our  race  and 
our  civilization  as  if  they  were  necessarily  su- 
preme and  immortal  it  is  well  to  remember 
that  there  have  been  other  races  and  other 
civilizations  that  regarded  themselves  in  the 
same  way.  "Assyria,  Greece,  Rome,  Carth- 
age, where  are  they?"  And  what  assurance 
have  we  that  our  race  and  our  civilization  will 
not  run  a  similar  course  and  come  to  a  similar 
end?  May  we  not  surely  predict  that  if  we 
continue  to  put  individual  freedom  and  luxury 
and  selfishness  above  social  obligations  our 
race  and  civilization  will  also  see  the  writing 
on  the  wall,  "Thou  are  weighed  in  the  bal- 
ances and  art  found  wanting"?  In  these  days 
when  individuals  are  demanding  more  and 
more  freedom  it  is  well  to  remember  that  "the 


GENETICS  AND  ETHICS  487 

best  use  that  man  has  made  of  his  freedom  has 
been  to  place  limitations  upon  it."  Again  and 
again,  age  after  age,  men  and  families  and 
nations  have  gone  up  to  a  climax  of  greatness 
and  then  have  declined,  while  other  unknown 
men  have  taken  their  places.  Greatness  has 
not  for  long  perpetuated  itself.  An  epitome 
of  human  history  is  contained  in  the  words, 
"He  hath  put  down  the  mighty  from  their 
seats  and  hath  exalted  them  of  low  degree." 

It  may  well  be  asked  by  those  who  are  in- 
terested in  breeding  a  better  race  of  men 
whether  such  a  thing  is  possible,  whether  the 
better  race  may  not  be  lacking  in  vitality  or 
fertility  or  morality  and  thus  be  doomed  to 
an  early  end.  Although  this  has  been  the  fate 
of  many  gifted  races  of  the  past  I  do  not  think 
that  it  was  a  necessary  fate.  The  history  of 
domesticated  animals  and  of  cultivated  plants, 
and  especially  the  recent  notable  advances  in 
genetics,  indicate  what  eugenics  might  do  for 
the  human  race.  In  time,  under  intelligent 
guidance,  the  worst  qualities  of  the  race  might 
be  weeded  out  and  the  best  qualities  preserved. 
This  is  the  goal  toward  which  intelligent  effort 
should  be  directed.  This  should  be  the  su- 


488  HEREDITY  AND  ENVIRONMENT 

preme  duty  of  society  and  of  all  who  love  their 
fellow  men. 

But  I  think  that  notable  human  improve- 
ment can  take  place  only  upon  two  conditions : 

(1)  The  physical  and  intellectual  improve- 
ment of  the  individual  through  environment 
and  training  must  not  interfere  with  his  racial 
and  ethical  obligations.     Individual  freedom 
must    be    subordinated    to    racial    welfare. 

(2)  The  promotion  of  human  evolution  must 
be  undertaken  by  society  as  its  greatest  work. 
Not   only  has   society  greater   freedom  and 
greater  power  than  the  individual  but  it  per- 
sists while  men  come  and  go. 

Our  hereditary  lines  are  so  interwoven  with 
those  of  other  races  and  will  be  so  entangled 
with  other  lines  in  the  future  that  any  selfish  or 
narrow  policy  of  improving  our  family  or  class 
can  have  little  permanent  value.  We  shall  rise 
only  as  the  race  rises.  Indeed  when  we  con- 
sider all  the  influences  of  our  fellow  men  upon 
our  development,  when  we  consider  our  he- 
reditary connections  with  multitudes  of  men 
and  women  of  the  past,  when  we  think  of  the 
nexus  of  hereditary  strands  which  are  woven 


GENETICS  AND  ETHICS  489 

into  our  personalities  and  which  will  be  con- 
tinued through  us  to  many  future  generations, 
we  realize  that  after  all  the  individual  is  not 
really  a  separate  and  independent  being,  but 
a  minor  unit  in  the  great  organism  of  hu- 
manity, and  that  his  greatest  duty  is  to  trans- 
mit unimpaired  and  undefiled  a  noble  heritage 
to  generations  yet  unborn. 

It  is  possible  greatly  to  improve  environ- 
ment. Conditions  of  life  are  still  hard  and 
cruel  for  many.  A  vast  amount  of  good  hu- 
man material  is  wasted  in  modern  society.  As 
civilization  becomes  more  complex  the  quan- 
tity of  human  wreckage  and  garbage  ever 
grows  greater.  Many  useful  lives  and  some 
great  possibilities  are  blotted  out  by  unfavor- 
able environment.  It  is  the  duty  of  society 
as  far  as  possible  to  conserve  these  lives  and 
to  develop  these  possibilities. 

It  is  possible  greatly  to  improve  education, 
to  make  it  a  potent  factor  in  development  in- 
stead of  a  conventional  veneer.  In  spite  of 
innumerable  educational  reforms  the  essential 
reform  has  not  yet  been  reached;  mere  refine- 
ments of  bad  methods  are  not  real  reforms. 


490  HEREDITY  AND  ENVIRONMENT 

The  essence  of  all  education  is  self  discovery 
and  self  control.  When  education  helps  an 
individual  to  discover  his  own  powers  and  limi- 
tations and  shows  him  how  to  get  out  of  his 
heredity  its  largest  and  best  possibilities  it  will 
fulfil  its  real  function;  when  children  are 
taught  not  merely  to  know  things  but  particu- 
larly to  know  themselves,  not  merely  how  to 
do  things  but  especially  how  to  compel  them- 
selves to  do  things,  they  may  be  said  to  be 
really  educated.  For  this  sort  of  education 
there  is  demanded  rigorous  discipline  of  the 
powers  of  observation,  of  the  reason,  and  es- 
pecially of  the  will. 

It  is  possible  greatly  to  improve  heredity: 
(a)  By  weeding  out  from  the  possibility  of 
reproduction  human  stocks  bearing  serious 
defects.  (b)  By  cultivating  pride  in  good 
heredity  and  by  discouraging  voluntary  in- 
fertility on  the  part  of  those  who  have  a  goodly 
heritage,  (c)  By  increasing  opportunities  for 
early  and  favorable  marriages,  (d)  By  care- 
fully conserving  the  best  human  mutations  or 
inherited  variations.  In  this  way  if  in  any 
way  the  better  race  will  be  produced.  The 


GENETICS  AND  ETHICS  491 

possible  improvements  of  heredity  are  great, 
the  possible  improvements  of  environment  and 
training  are  great,  but  whether  men  of  the  fu- 
ture will  be  better  than  those  of  the  past  or 
present  is  a  question  not  only  of  genetics  but 
also  of  ethics. 

How  better  can  I  close  this  course  of  lec- 
tures than  with  the  words  of  Francis  Galton, 
one  of  the  greatest  students  of  human  he- 
redity and  the  founder  of  the  science  of  Eu- 
genics ? 

"The  chief  result  of  these  inquiries  has  been  to 
elicit  the  religious  significance  of  the  doctrine  of 
evolution.  It  suggests  an  alteration  in  our  mental 
attitude  and  imposes  a  new  moral  duty.  The  new 
mental  attitude  is  one  of  a  greater  sense  of  moral 
freedom,  responsibility  and  opportunity;  the  new 
duty  which  is  supposed  to  be  exercised  concurrently 
with,  and  not  in  opposition  to  the  old  ones  upon 
which  the  social  fabric  depends,  is  an  endeavor  to 
further  evolution,  especially  that  of  the  human  race." 


REFERENCES  TO  LITERATURE 

The  following  list  of  books  and  publications  in- 
cludes only  those  works  which  are  referred  to  most 
frequently  in  the  preceding  pages.  Several  of  the 
books  cited,  particularly  those  by  Plate  and  Morgan, 
contain  extensive  bibliographies.  Those  desiring  to 
become  more  fully  acquainted  with  books  and  articles 
dealing  with  the  subjects  of  heredity  and  develop- 
ment are  referred  to  the  larger  works  which  are  listed 
here. 

Books  and  Larger  Works 

Bateson,  W.  Materials  for  the  Study  of  Variation, 
London,  1894. 

Bateson,  W.  Problems  in  Genetics.  Yale  Univ. 
Press.  1913. 

Bateson,  W.  Mendel's  Principles  of  Heredity.  3rd 
Impression,  Cambridge,  1913. 

Baur,  E.  Einfiihrung  in  die  experimented  Verer- 
bungslehre.  Berlin,  1911. 

Castle,  W.  Heredity  in  Relation  to  Evolution  and 
Animal  Breeding.  New  York,  1912. 

Castle,  W.  E.;  Coulter,  J.  M. ;  Davenport,  C.  B. ; 
East,  E.  M.;  Tower,  W.  L.  Heredity  and  Eu- 
genics. Chicago,  1912. 

Correns,  C.  Die  Neuen  Vererbungsgesetze.  Berlin, 
1912. 

Darbishire,  A.  R.  Breeding  and  Mendelian  Discov- 
ery. London,  1911. 

493 


494  HEREDITY  AND  ENVIRONMENT 

Darwin,  C.  Animals  and  Plants  under  Domestica- 
tion. New  York,  1887. 

Davenport,  C.  B.  Heredity  in  Relation  to  Eugen- 
ics. New  York,  1911. 

De  Vries,  H.  Intracellular  Pangenesis.  Chicago, 
1910. 

De  Vries,  H.     Die  Mutationstheorie.     Leipzig,  1901. 

De  Vries,  H.    Plant  Breeding.    Chicago,  1907. 

Doncaster,  L.  Heredity  in  the  Light  of  Recent  Re- 
search. Cambridge,  1911. 

Driesch,  H.  The  Science  and  Philosophy  of  the  Or- 
ganism. Gifford  Lectures,  London,  1908. 

Ellis,  H.  The  Task  of  Social  Hygiene.  London, 
1912. 

Ellis,  H.  The  Problem  of  Race  Regeneration.  New 
York,  1911. 

Forel,  A.    The  Sexual  Question.     New  York,  1908. 

Galton,  F.  Inquiries  into  Human  Faculty.  New 
York,  1883. 

Galton,  F.     Natural  Inheritance.     London,  1889. 

Galton,  F.     Hereditary  Genius.     London,  1892. 

Galton,  F.     Essays  in  Eugenics.     London,  1909. 

Goddard,  H.  H.  The  Kallikak  Family.  New  York, 
1912. 

Goldschmidt,  R.  Einfiihrung  in  die  Vererbungswis- 
senschaft.  Leipzig,  1911. 

Hacker,  V.  Allgemeine  Vererbungslehre.  Braun- 
schweig, 1912. 

Hertwig,  O.     Allgemeine  Biologie.     Jena,  1909. 

Johannsen,  W.  Elemente  der  exakten  Erblichkeits- 
lehre,  2d  Auf.  Jena,  1913. 

Kellicott,  W.  E.  The  Social  Direction  of  Human 
Evolution.  New  York,  1911. 

Lock,  R.  H.  Variation,  Heredity  and  Evolution. 
New  York,  1911. 


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Loeb,  J.  Comparative  Physiology  of  the  Brain  and 
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Loeb,  J.  The  Dynamics  of  Living  Matter.  New 
York,  1906. 

Loeb,  J.  The  Mechanistic  Conception  of  Life. 
Chicago,  1911. 

Loeb,  J.    Artificial  Parthenogenesis.     Chicago,  1913. 

Mctchnikoff,  E.  The  Nature  of  Man.  Chicago, 
1903. 

Morgan,  T.  H.  Heredity  and  Sex.  New  York, 
1913. 

Mott,  F.  W.  Heredity  and  Eugenics  in  Relation  to 
Insanity.  London,  1912. 

Nageli,  C.  Mechanische-Physiologische  Theorie  der 
Abstammungslehre.  Miinchen,  1884. 

Plate,  L.     Vererbungslehre.     Leipzig,  1913. 

Problems  in  Eugenics.  Papers  communicated  to  1st 
International  Eug.  Cong.  London,  1912. 

Punnett,  R.  C.     Mendelism.     London,  1911. 

Rignano,  E.  The  Inheritance  of  Acquired  Charac- 
ters. Chicago,  1911. 

Romanes,  G.  J.  Darwin  and  After  Darwin.  Chi- 
cago, 1892. 

Saleeby,  C.  W.  Parenthood  and  Race  Culture.  New 
York,  1909. 

Semon,  R.  Das  Problem  der  Vererbungslehre  er- 
worbener  Eigcnschaften.  Leipzig,  1912. 

Spencer,  H.  Principles  of  Biology.  New  York, 
1883. 

Thompson,  J.  A.     Heredity.     Edinburgh,  1908. 

Thorndike,  E.  L.  Animal  Intelligence.  New  Y'ork, 
1911. 

Treasury  of  Human  Inheritance.     London,  1912. 

Walter,  H.  E.     Genetics.     New  York,  1913. 


496  HEREDITY  AND  ENVIRONMENT 

Weismann,  A.     The  Germ  Plasm.     New  York,  1893. 

Weismann,  A.     Essays  on  Heredity.     Oxford,  1889. 

Wilson,  E.  B.  The  Cell  in  Development  and  In- 
heritance. New  York,  1900. 

Woods,  F.  A.  Heredity  in  Royalty.  New  York, 
1906. 


Monographs  and  Papers 

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Roentgen  Rays.     Jour.  Exp.  Zool.,  4,  1907. 
Baur,  E.     Vererbungs  und  Bastardierungsversuche 

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Boveri,  Th.     Zellen  Studien.     Die  Entwicklung  di- 

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1911. 


GLOSSARY 

ACCESSORY  CHRO'-MO-SOME.  An  odd  chromosome  which  is  found 
in  only  half  of  the  spermatozoa  of  certain  animals;  see 
"sex-chromosome." 

A-CHRO'-MA-TIN.  The  non-staining  substance  of  the  nucleus 
as  contrasted  with  the  chromatin. 

A-CHON'-DRO-PLA-SY.  A  condition  in  which  the  long  bones 
cease  to  grow  in  length  at  an  early  age  thus  producing  a 
dwarf  with  large  body  and  head  but  short  limbs. 

ACQUIRED  CHARACTER.  A  character,  the  differential  cause  of 
which  is  environmental. 

ALTERNATIVE  INHERITANCE.  Galton's  term  for  a  doubtful  kind 
of  inheritance  in  which  all  characters  are  derived  from  one 
parent.  In  present  use,  Mendelian  inheritance. 

AM'-NI-ON.  One  of  the  embryonic  membranes  of  higher  verte- 
brates. 

AM-PHI-OX'-US.  One  of  the  lowest  and  simplest  animals  hav- 
ing a  notochord  (backbone). 

AN-EN-CEPH'-A-LT.    The  condition  of  a  brainless  monster. 

ANIMAL  POLE.  That  pole  of  an  egg  at  which  the  polar  bodies 
are  formed. 

AN'-LA-GE.     The  embryonic  basis  of  any  developed  part. 

A-OR'-TA.     The  great  artery  arising  from  the  heart. 

AR'-CHI-PLASM.  The  deeply  staining  plasm  surrounding  the 
centrosome. 

AS'-CA-RIS.  A  genus  of  round  worms  which  are  intestinal 
parasites. 

AS'-CA-RIS  meg-a-lo-ceph'-a-la.  A  parasite  in  the  intestine  of 
the  horse. 

AS-CID'-I-AN.     A  "sea-squirt";  one  of  the  lowest  types  having 
a  notochord,  or  elementary  backbone. 
503 


504  GLOSSARY 

AS'-TER.  The  radiating  figure  surrounding  the  centrosome  in 
a  cell. 

AS-SIM-I-LA'-TION.  Conversion  of  food  substances  by  an  or- 
ganism into  its  own  living  substance. 

A-SYM'-ME-TRY.  The  condition  where  opposite  sides  are 
unlike. 

AT'-A-VISM.  The  condition  in  which  an  individual  resembles  a 
grandparent,  or  a  more  distant  ancestor,  more  than  one 
of  the  parents. 

BI'-O-PHOKES.     The  ultimate  units  of  life   (Weismann). 

BI'-VA-LENT  CHRO'-MO-SOMES.  A  pair  of  chromosomes,  one  ma- 
ternal the  other  paternal,  temporarily  united. 

BLAS'-TO-COEL.     The  cavity  within  a  blastula. 

BLAS'-TO-DER'-MIC  VES'-I-CLE.  A  hollow  sphere,  formed  from 
the  segmented  egg  of  a  mammal,  which  becomes  attached 
to  or  embedded  within  the  wall  of  the  uterus. 

BLAS'-TO-PORE.     The  mouth  of  a  gastrula. 

BLAS'-TTJ-LA.  A  mass  of  cells,  usually  in  the  shape  of  a  hollow 
sphere,  formed  by  repeated  divisions  (cleavages)  of  an 

egg- 

BLENDING  INHERITANCE.  Galton's  term  for  that  kind  of  in- 
heritance in  which  the  characters  of  the  parents  seem  to 
blend  in  the  offspring. 

BRACH-Y-DAC'-TY-LISM.  The  condition  of  having  abnormally 
short  fingers  or  toes. 

CELL.  The  fundamental  unit  of  structure  and  function  in  all 
living  things. 

CEN'-TRO-SOME.  The  body  at  the  center  of  radiations  in  a 
dividing  cell. 

CEPH'-A-LO-PODS.  A  class  of  mollusks  which  includes  the  squid, 
cuttle-fish  and  devil-fish. 

CER'E-BRAL  GANO'-LI-ON.  The  brain  of  an  invertebrate 
animal. 

CHARACTER.     Any  feature  or  property  of  an  organism. 

CHOR'-DA.  A  cellular  rod  in  vertebrate  embryos  which  forms 
the  basis  of  the  backbone. 

CHOR'-DATE.  A  member  of  the  highest  phylum  of  the  animal 
kingdom,  including  all  animals  having  a  chorda  or  back- 
bone. 


GLOSSARY  505 

CHO'-RI-ON.     A   tough   membrane   around   an  egg  secreted   by 

.surrounding  cells. 

CHRO'-MA-TIK.     The  deeply  staining  substance  of  the  nucleus. 
CHHO'-MO-SOMES.     Deeply  staining  bodies  found  in  the  nucleus 

at  the  time  of  indirect  division. 
CII/-I-A.     Minute  protoplasmic  threads  on  the  surface  of  a  cell 

which  produce  movements  in  the  surrounding  medium  by 

waving  back  and  forth. 
CLASS.     The  chief  sub-division  of  a  phylum. 
CLEAV'-AGE.     The  division  of  the  egg  cell  after  fertilization  into 

many  cells. 

CLEP-SI'-NE.     A  genus  of  leeches. 
COE'-LOM.     The  body  cavity. 

CONTINUOUS  VARIATION.     A  series  of  minute  variations. 
CORRELATIVE    DIFFERENTIATION.     Differentiation    due    chiefly    to 

the  interaction  of  different  parts  of  an  organism. 
CRE-PID'-U-LA.     A  genus  of  marine  gastropods. 
"CRISS-CROSS"   INHERITANCE.     Morgan's  term   for  that  kind  of 

inheritance  in  whicn  maternal  characters  are  transmitted 

to  sons  and  paternal  ones  to  daughters. 
CTEN'-O-PHORE.     A   jelly-sphere;    a   member   of   a   phylum    of 

marine  animals  standing  above  the  jelly-fishes. 
CY-CLO'-PI-A.     A   monstrosity   in   which   both   eyes   have   fused 

into  a   single  one. 

CY-TOL'-O-GY.     The  science  which  treats  of  cells. 
CY'-TO-PLASM.     The  protoplasm  of  a  cell  outside  of  the  nucleus. 
DAL'-TON-ISM.     That   form  of  color-blindness  in   which  one  is 

unable   to   distinguish   red   and   green;   usually   limited   to 

males. 
DAR' -WIN-ISM.     The  doctrine  that  evolution  takes  place  through 

natural  selection  or  the  survival  of  the  fittest. 
DETERMINANTS.     The  units   of  heredity    (Weismann). 
DETERMINER.     The  differential  cause  or  factor  in  a  germ  cell 

which  determines  the  development  of  a  character. 
DEX'-THAL  SNAIL.     The  usual  type  of  snail  in  which  the  shell 

coils  from  base  to  apex  in  a  clockwise  direction. 
DIFFERENTIATION.     The  process  of  producing  specific  parts  or 

substances  from  a  general  part  or  substance. 


506  GLOSSARY 

DI-HY'-BRID.  The  offspring  of  parents  differing  in  two  char- 
acters. 

DI-O-NAE'-A.     An  insect-catching  plant,  the  "Venus  Fly-trap." 

DIP'-LOID.  The  full  number  of  chromosomes  found  in  the  ferti- 
lized egg  and  in  all  cells  derived  from  this,  except  the 
mature  germ  cells. 

DOMINANT  CHARACTER.  A  character  inherited  from  one  parent 
which  develops  to  the  exclusion  of  a  contrasting  character 
of  the  other  parent. 

DROS-OPH'-I-LA.     A  genus  of  fruit-flies. 

DU'-PLEX  FACTORS  or  CHARACTER.  A  condition  where  the  de- 
terminers for  a  character  are  derived  from  both  parents. 

E-CHI'-NO-DERMS.  A  phylum  of  marine  animals  which  includes 
star-fishes  and  sea-urchins. 

E-COL'-O-GY.  The  science  which  deals  with  the  relations  of 
organisms  to  one  another  and  to  environment. 

EC'-TO-DERM.  The  outer  layer  of  cells  of  an  embryo  which 
gives  rise  to  epidermis,  sense  organs  and  nervous  system. 

EM-BRY-OG'-E-NY.  Early  development  of  an  egg  leading  to 
the  formation  of  an  embryo. 

EN'-DO-DEHM.  The  inner  layer  of  cells  of  an  embryo,  which 
gives  rise  to  the  digestive  cells  of  the  alimentary  system. 

EP-I-GEN'E-SIS.  The  doctrine  that  the  germ  is  simple  and 
homogeneous  and  that  development  consists  in  the  for- 
mation of  complex  parts  from  the  simple  germ. 

EQUATION-DIVISION.  An  ordinary  nuclear  division  in  which 
each  chromosome  divides  equally. 

EU-GEN'-ICS.    The  system  of  imp  roving 'races  by  good  breeding. 

EU-THEN'-ICS.  The  system  of  improving  individuals  by  good 
environment. 

EX-O-GAS'-TRU-LA.  A  gastrula  with  the  endoderm  turned  out 
instead  of  in. 

FACTOR.    A  specific  germinal  cause  of  a  developed  character. 

FERTILIZATION.    The  union  of  male  and  female  sex  cells. 

FLA-GEL'-LUM.  A  vibratile  thread  of  protoplasm  which  serves 
as  an  organ  of  locomotion. 

FLUCTUATIONS.     Variations  which  are  not  inherited. 


GLOSSARY  507 

FOL'-LI-CLE  CELLS.     Nutritive  cells  surrounding  an  ovarian  egg. 

FRATERNAL  TWINS.  Twins  produced  from  different  eggs  and 
showing  different  hereditary  characters. 

FUNCTIONAL  ACTIVITY.     Use. 

GAM'-ETE.    The  mature  male  or  female  sex  cell. 

GANG'-LI-ON.    A  group  of  nerve  cells. 

GAS'-TEO-COEL.     The  digestive  cavity  of  the  gastrula. 

GAS'-TRU-LA.  A  stage  in  development  following  the  blastula,  in 
which  the  embryo  consists  of  an  outer  (ectoderm)  and  an 
inner  (endoderm)  layer  of  cells. 

GENES.  Factors,  units,  elements  of  germ  cells  which  condition 
the  characters  of  developed  organisms  (Johannsen). 

GE-NET'-ICS.  The  science  which  deals  with  the  origin  of  indi- 
viduals and  particularly  with  heredity. 

GE'-NO-TYPE.  The  germinal  type  with  all  its  hereditary  pecu- 
liarities. "The  fundamental  hereditary  constitution  of  an 
organism"  (Johannsen). 

GERM-PLASM.     The  material  basis  of  inheritance. 

GERM-TRACK.  The  cell-lineage  of  the  germ  cells  in  a  develop- 
ing animal. 

GERMINAL  UNITS.  Hypothetical  parts  of  germ  cells  which  are 
supposed  to  have  certain  specific  functions  in  development. 

HAE-MO-PHIL'-I-A.  An  abnormal  condition  in  which  the  blood 
clots  slowly. 

HAP'-LOID.  The  reduced  number  of  chromosomes  in  the 
gametes. 

HEREDITY.  The  appearance  in  offspring  of  characters  whose 
differential  causes  are  in  the  germ  cells. 

HERITAGE.  The  sum  of  those  characters  which  are  inherited  by 
an  individual. 

HET-ER-O-ZY-GO'-SIS.     Hybridization ;  cross-breeding. 

HET-ER-O-ZY'-GOTES.  Hybrids  resulting  from  the  union  of 
gametes  which  are  hereditarily  dissimilar. 

HO-MO-ZY'-GOTES.  Pure-breds  resulting  form  the  union  of 
gametes  which  are  hereditarily  similar. 

HY'-BRID.  The  offspring  of  parents  which  differ  in  one  or  more 
characters. 


508  GLOSSARY 

IDENTICAL  TWINS.  Twins  which  have  come  from  a  single  egg 
and  which  show  identical  hereditary  characters. 

ID'-I-O-PLASM.     The  germ-plasm  or  inheritance  material. 

INDUCTION.  A  modification  of  the  first  filial  generation  caused 
by  the  action  of  environment  on  the  germ  cells  of  the 
parental  generation. 

INHERITED  CHARACTER.  A  character  the  differential  cause  of 
which  is  in  the  germ. 

INSTINCTS.     Complex  reflexes  involving  nerve  centers. 

INVERSE  SYMMETRY.  Having  the  right  half  of  one  asymmetri- 
cal individual  equivalent  to  the  left  half  of  another. 

IRRITABILITY.     Capacity  of  receiving  and  responding  to  stimuli. 

LA-MARCK'-ISM.  The  doctrine  that  evolution  takes  place 
through  the  inheritance  of  acquired  characters. 

LOCALIZATION.  The  gathering  together  of  particular  sub- 
stances in  definite  parts  of  an  egg  or  embryo. 

LOL'-I-GO.     The  squid,  a  genus  of  cephalopod  mollusks. 

MAR-SU'-PI-ALS.  A  primitive  group  of  mammals,  including  op- 
posums  and  kangaroos,  which  carry  the  young  in  a  pouch. 

MAT-u-RA'-TfoN.  The  final  stages  in  the  formation  of  sex  cells, 
characterized  by  two  peculiar  cell  divisions. 

ME-RIS'-TIC  VARIATION.     Variation  in  the  number  of  parts. 

MES'-EN-CHYME.     Loosely  scattered  cells  of  the  mesoderm. 

MES'-O-DERM.  A  layer  or  group  of  embryonic  cells  lying  be- 
tween ectoderm  and  endoderm. 

ME-TAB'-O-LISM.  Transformations  of  matter  and  energy  within 
a  living  thing. 

MI'-CRO-PYLE.  The  minute  opening  in  an  egg  membrane 
through  which  the  spermatozoon  enters. 

MI-TO'-SIS.  Indirect  nuclear  division  in  which  the  nucleus  is 
transformed  into  a  spindle  and  chromosomes;  the  latter 
split  and  the  halves  move  to  the  poles  of  the  spindle 
where  they  form  the  daughter  nuclei. 

MON-O-HY'-BRID.  The  offspring  of  parents  differing  in  one 
character. 

MON'-O-TREMES.  The  lowest  group  of  mammals,  including  the 
duck-bill  and  the  spiny  anteater. 


GLOSSARY  509 

MOR-PHOL'-O-GY.     The   science  which  deals  with  structure  and 

form. 

MUS'-CA.     A  genus  of  flies  including  the  house-fly. 
MU'-TANT.     A  sudden  variation  or  sport  which  breeds  true. 
MU-TA'-TIONS.     Inherited    variations    which    are    more    or    less 

striking. 

NEC-TU'-RUS.     A  large  salamander;  the  mud-puppy. 
NEM'-A-TODE.     A   round-worm  or  thread-worm. 
NE'-HE-IS.     A  marine  annelid,  or  ringed  worm. 
NEURAL    GROOVE.     The   groove   on    the    dorsal    surface    of    the 

embryo  of  a  vertebrate  which  develops  into  the  brain  and 

spinal  cord. 
NEURAL   TUBE.     A   tube   formed   from   the   neural   groove   and 

giving  rise  to  brain  and  spinal  cord. 
NO'-TO-CHORD.     The  cellular  rod  which  forms  the  basis  of  the 

backbone. 
NU'-CLE-US.     The  central  organ   of  a  cell,  composed  of  chro- 

matin  and  achromatin. 
NULLIPLEX   FACTORS   or   CHARACTER.     A   condition   in   which    a 

character    is    absent   because    its    determiner    is    found    in 

neither  parent. 

ON-TOG'-E-NY.     Development  of  an  individual. 
O'-O-CYTE.     The  ovarian  egg  before  maturation   (formation  of 

polar  bodies). 
O-O-GEN'-E-SIS.     The  development  of  an  ovum  from  a  primitive 

sex-cell. 
O-O-GO'-NI-A.     The  earliest  generations  of  cells  which  produce 

ova;  primordial  egg  cells. 

O'-O-SPERM.     The  fertilized  egg  after  union  of  egg  and  sperm. 
ORDER.     The  chief  sub-division  of  a  class. 
ORGANIZATION.     Differentiation    and    integration,    i.e.    different 

parts  united  into  one  whole. 
OR-GAN-OG'-E-XY.     The    formation    of    various    organs    of    the 

body. 
OR-THO-GEN'-E-SIS.     The  doctrine  that  the  course  of  evolution 

is   definitely  directed  by  intrinsic  causes. 


510  GLOSSARY 

O-VI-PAR'-I-TY.  Young  brought  forth  as  eggs,  i.e.,  in  an  early 
stage  of  development. 

O'-VULES.  The  female  sex  cells  of  flowering  plants  with  the 
immediately  surrounding  parts. 

O'-VUM.    The  female  sex  cell. 

OX-Y-CHRO'-MA-TIN.  That  portion  of  the  chromatin  which 
does  not  form  chromosomes. 

PAN-GEN'-E-SIS.  The  hypothesis  proposed  by  Darwin  that  every 
cell  of  the  body  gives  off  minute  germs,  "gemmules,"  which 
then  collect  in  the  sex  cells. 

PAR-A-ME'-CI-UM.    A  ciliated  protozoan. 

PAR-THE-NO-GEN'-E-SIS.  Development  of  an  egg  without  pre- 
vious fertilization. 

PARTICULATE  INHERITANCE.  Gallon's  term  for  that  kind  of  in- 
heritance in  which  certain  characters  are  derived  from  one 
parent  and  others  from  the  other  parent,  i.e.  Mende- 
lian  Inheritance. 

PA-THOL'-O-GY.     The  science  which  deals  with  disease. 

PHE'-NO-TYPE.  The  developed  type  in  which  some  of  the  her- 
editary possibilities  are  realized1  while  others  remain  un- 
developed. "Developed,  measurable  realities"  (Johannsen). 

PHY-LOG'-E-NY.     Evolution  of  a  race  or  species. 

PHYL-LOX'-E-RA.     A  genus  of  plant  lice. 

PHY'-LUM.  One  of  the  chief  sub-divisions  of  the  animal  king- 
dom. 

PHYS-I-OL'-O-GY.    The   science  which  deals   with   function. 

PLAS'-TO-SOMES.  Threads  or  granules  in  the  cytoplasm  which 
are  colored  by  certain  dyes. 

POLAR  BODIES.  Two  minute  cells  which  are  separated  from 
the  egg  in  its  two  maturations  divisions. 

PO-LAR'-I-TY.  The  condition  where  two  poles  of  a  body  differ; 
in  eggs  the  two  poles  are  the  animal  (formative)  and  the 
vegetative  (nutritive) . 

POL'-LEN.    The  male  sex  cells  of  flowering  plants. 

POL-Y-DAC'-TYL-ISM.  The  condition  of  having  more  than  the 
normal  number  of  digits  on  hands  or  feet. 

POL-Y-HY'-BRID.  The  offspring  of  parents  differing  in  more 
than  three  characters. 


GLOSSARY  511 

PRE-FOR-MA'-TION.  The  doctrine  that  the  fully  formed  organ- 
ism exists  in  the  germ,  and  that  development  is  merely 
its  unfolding. 

PRE-IN-DUC'-TION.  A  modification  of  the  second  filial  gener- 
ation caused  by  the  action  of  environment  on  the  germ 
cells  of  the  parental  generation. 

PRE-PO'-TEN-CY.  The  preponderance  of  one  parent  over  the 
other  in  the  transmission  of  hereditary  characters. 

PRI'-MATES.  The  highest  order  of  mammals,  including  monkeys, 
apes,  and  man. 

PRIMITIVE  SEX  CELLS.  The  earliest  recognizable  progenitors 
of  the  sex  cells  in  development. 

PRO'-TE-IN.  Complex  organic  substances  containing  nitrogen, 
e.g.  white  of  egg. 

PRO-TE'-NOR.     A  genus  of  the  true  bugs. 

PRO'-TO-PLASM.     The  living  material  of  an  organism. 

PRO-TO-ZO'-A.  The  simplest  animals,  usually  consisting  of  a 
single  cell. 

PY-LO'-RUS.  The  narrow  opening  between  stomach  and 
intestine. 

RECESSIVE  CHARACTER.  An  inherited  character  which  remains 
undeveloped  when  mated  with  a  dominant  character. 

REDUCTION-DIVISION.  That  maturation  division  in  which  the 
number  of  chromosomes  is  halved. 

REFLEXES.     Relatively  simple,  automatic  responses. 

RESPONSE.  Any  activity  of  an  organism  called  forth  by  a 
stimulus. 

REVERSIONS.  The  sudden  reappearance  of  long-lost  racial 
characters. 

SEGREGATION.  The  separation  of  dominant  and  recessive  char- 
acters in  the  offspring  of  hybrids. 

SELF  DIFFERENTIATION.  Differentiation  due  chiefly  to  intrinsic 
causes. 

SENSITIVITY.     Capacity  of  receiving  and  responding  to  stimuli. 

SEX  CHRO'-MO-SOME.  The  "odd"  or  accessory  chromosome 
which  is  supposed  to  determine  sex. 

SEX-LIMITED.    Any  character  which  is  found  in  one  sex  only. 


512  GLOSSARY 

SEX-LINKED.  Any  character,  the  determiner  of  which  is  as- 
sociated with  the  determiner  of  sex. 

SIMPLEX  FACTORS  or  CHARACTER.  A  condition  where  the  de- 
terminer for  a  character  is  derived  from  one  parent  only. 

SIN'-IS-TRAL  SNAIL.  A  type  of  snail  in  which  the  shell  coils 
from  base  to  apex  in  an  anti-clockwise  direction. 

SO'-MA.    The  body  as  contrasted  with  the  germ  cells. 

SO-MAT' -ic.  Pertaining  to  the  body,  as  contrasted  with  "germi- 
nal" pertaining  to  the  germ  cells. 

SO'-MA-TO-PLASM.  The  body-plasm  as  contrasted  with  the 
germ-plasm. 

SO'-MITE.     A  segment  of  the  body  of  a  segmented  animal. 

SPER-MA'-TO-CYTES.  The  mother  and  grandmother  cells  of 
spermatozoa. 

SPER-MA-TO-GEN'-E-SIS.  The  development  of  a  spermatozoon 
from  a  primitive  sex  cell. 

SPER-MA-TO-GO'-NI-A.     Primordial  sperm  cells. 

SPER-MA-TO-ZO'-ON.     The  mature  male  sex  cell. 

SPINDLE.     The  nuclear  division  figure. 

SPI'-REME.  A  coiled  thread  of  chromatin  which  appears  in  the 
nucleus  at  the  beginning  of  division. 

SPI-RIL'-LA.     A  spiral  type  of  bacteria. 

STEN'-TOR.     A  ciliated  protozxmn. 

STER-E-O-I'-SO-MERES.  Molecules  having  different  properties 
dependent  upon  varying  spacial  relations  of  their  con- 
stituent atoms. 

STIM'-U-LUS.  Anything  acting  on  an  organism  which  calls 
forth  a  response. 

STY-E'-LA.     A  genus  of  Ascidians. 

SYM'-ME-THY.  The  condition  where  opposite  sides  or  poles  are 
alike;  bilateral,  having  equivalent  right  and  left  sides. 

SYN-AP'-SIS.  The  conjugation  of  maternal  and  paternal  chro- 
mosomes preceding  the  maturation  divisions. 

SYN-DAC'-TYL-ISM.  The  condition  of  having  webbed  fingers  or 
toes. 

TE-NEB'-RI-O.  A  genus  of  beetles,  the  larva  of  which  is  the 
common  meal  worm. 


GLOSSARY  513 

TER-A-TOL'-O-GY.  The  science  which  deals  with  monstrous  or 
abnormal  forms. 

TET'-RADS.  Bivalent  chromosomes  which  appear  4-parted  in 
the  maturation  divisions. 

TO-TIP'-O-TENCE.  The  capacity  of  a  cleavage  cell  to  give  rise 
to  a  whole  animal. 

TOX'-IN.  A  poisonous  substance  particularly  such  as  is  pro- 
duced by  bacteria. 

TRI-HY'-BRID.  The  offspring  of  parents  differing  in  three  char- 
acters. 

TROPH'-O-BLAST.  The  outer  layer  of  the  blastodermic  vesicle 
of  a  mammal. 

TRO'-PISMS.  Automatic  movements  of  organisms  toward  or 
away  from  a  source  of  stimulus. 

UNIT  CHARACTER.  A  character  which  is  inherited  as  a  whole 
and  cannot  be  sub-divided. 

VEGETATIVE  POLE.  The  pole  of  an  egg  opposite  the  polar 
bodies. 

VIL'-LI.  Processes  which  grow  out  from  the  embryonic  mem- 
branes of  a  mammal  and  connect  it  to  the  walls  of  the 
uterus. 

VI-TEL'-LIKE  MEMBRANE.  A  delicate  membrane  around  an  egg 
secreted  by  the  egg  itself. 

VIV-I-PAR'-I-TY.  Young  brought  forth  "alive,"  in  an  advanced 
stage  of  development. 

ZY'-GOTE.  The  product  of  the  union  of  male  and  female  sex 
cells. 


INDEX 

Proper  names  and  titles  of  sections  are  in   small  capitals; 
page  references  to  illustrations  in  italic  numerals. 


Aborigines   of  Australia  402 

New  Zealand  402 

North  America  402 

Pacific   Islands  402 

South  America  402 

Tasmania  401 

West  Indies  402 
"Accessory"    chromosome,    140, 

141,  143,  144,  145,  166,  167, 

271-277 

Achondroplasy,  200,  293,  294 
Achromatin,  109,  113 

differential          distribution, 

184,  in  cell  body,  181 
ACQUIRED  CHARACTERS,   INHERI- 
TANCE OF,  334-351 

Darwin   on,   335 

Lamarck    on,    334 

Weismann  on,  335 

definition   of,   336,   337 

general  objections  to,  338 

specific   objections  to,   339- 

347 

Adaptive  responses,  55,  78 
Adrenal  gland,  fed  to  tadpoles, 

326 
African  negro,  402,  407 

in  Jamaica,  417 

in  U.  S.,  417 
Age  of  human  race,  398 
Albinism,  289,  291,  292 
Alcoholism,  206,  295,  450 

cause  of  sterility,  still- 
births,  malformation, 
dwarfs,  311,  312,  313 

induction  effect  on  rotifers, 
349 

influence  on  germ  cells,  311 


Alkaptonuria,  293 
Alpine  plants,  non  inheritance 
of  acquired  characters,  345 
"Alternative"   inheritance,  208 
Alternatives     in     development, 

462,   463 
Amalgamation    of    races,    402, 

416-419 

American  men  of  science,  fami- 
lies  of,  4?5 

AMPHIOXUS,  cleavage  and  differ- 
entiation, 119 
cleavage    and    gastrulation, 

25,  larvae,  27,  29 
isolated  cleavage  cells,  314, 

S15 

Ancestors,  number  of,  216-218 
Ancestry,  pride  of,  429 
ANCYRACANTHUS,  oogenesis,  134, 

spermatogenesis,  132 
Andalusian  fowl,  Blue,  266 
Anencephaly,  325 
Animal  pole  of  egg,  167 
Annelid  type  of  egg,  178 
Ants  and  bees,  440,  483 
Artificial    limitation    of    fami- 
lies, 437,  438 

Artificial  parthenogenesis,  314 
ARTIFICIAL  SELECTION,  376 

chief   factor  in  production 

of  domestic  races,  376 
creates  nothing,  377-380 
isolates  pure  lines,  378,  379 
lacking,  409,  411,  412 
methods  of,  376,  377 
results  of,  377-380 
ASCARIS,   fertilization   of,  112 


515 


516 


INDEX 


"germ   track,"    126,   127 

sex  differentiation  in,  144. 
Ascidian  egg,  119,  120,  124,  168, 
317,  319,  320,  322 

type,  178 
ASSHETON,   cleavage   of  egg  of 

sheep,  23 
Aster,   109,   111 
Astral   radiations,   110 
Atavism,  209 
ATHENS,  406,  407 
Athletes,  prize,  415 
Athletics,  educational  value  of, 

476,  477 
ATTICA,  illustrious  men  of,  405- 

407 
Automaton,  459,  461 

Backbone,   development   of,   28, 

29 

"Back  cross,"  ratios  of,  237,  238 
Bacteria,  reactions  to  light,  49 
Baldness,  inherited,  203 
BALZAC,  heredity  a  maze,  297 
Barbarism,  363,  396,  411,  486 
BARDEEN,    X-rays    on    sperma- 
tozoa, 311,  496 

BATESON,  Blue  Andalusian,  266 
brachydactyl   hand,   292 
"homozygote  and  heterozy- 

gote,"  228 

on  Mendelism,  225,  493 
BATESON  and  PUNNETT,  on  sweet 
pea  hybrids,  256,  257,  258 
.  BATTR,    factors    for    flowers    of 

Antirrhinum,  259 
induction    effects    of    poor 

soil,  349,  493,  496 
Beans,  pure  lines  of,  197,  205 
Bees,  and  Ants,  440,  483 

influence  of  food  on  devel- 
opment,  326 
workers,     queens,     drones, 

326 

BEETHOVEN,  473 
Behavior,  dogs,  cats,  monkeys, 

fish  and  frog,  465 


lower  organisms,  44 
modifiability  of,  68 
Paramecium,    61-63,    66 
plasticity  of,  71,  464 
rigidity  of,   71 
test  of  psychical  processes, 

48 

worms,  star-fish,  Crustacea, 
vertebrates,  63,  64,  66,  70, 
71 

BERKELEY,  BISHOP,  446 
"Biophores"  of  Weismann,  100 
Birthrate    and    deathrate,    nor- 
mally equal,  432 
both  decreasing,  433,  434 
decreasing     most     in     best 

families,  434 
in    Massachusetts,    435 
Bivalent  chromosomes,   133 
Blastula,  25,  26 
"BLENDING"    INHERITANCE,   208, 

280,  282-287 

in  length  of  ears  in  rab- 
bits, 285 

in   length   of   skull   in   rab- 
bits, 285,  286 
in    skin    color    of    mulatto, 

282-285,  £88 
"Blood  lines,"  379 
BOND,  negro  X  white  cross,  288 
BOULE,       Chapelle  -  aux  -  Saints 

skull,  397 
BOVERI,    chromosomes    differ    in 

value,  156,  165 
dispermic  eggs,  313,  496 
Brachydactylism,  199,  292,  293 
Brain,  development,  28 

size  and  weight,  472,  473 
Breeder,  methods  of,  416 
BROMAN,  death  of  families,  436 

496 

BROOKS,  214,  496 
Buddhistic  belief  in  transmigra- 
tion,  40 
BURBANK,  new  combinations  of 

characters,  381 
mutations,  384 


INDEX 


517 


Canary  birds,   fed  on  red  pep- 
per, 326 

Capacity,  greater  than  realiza- 
tion, 471,  472 
Captivity,   cause    of   infertility, 

436,  437 
CASTLE,  493,  496 

factors    for   coat   colors   of 

rabbits,  259 

recombinations    o  f  charac- 
ters, 381,  382,  883 
size  in  rabbits,  285-286 
CASTLE     and     PHILLIPS,     trans- 
planted ovaries  of  guinea- 
pigs,  342-^,  346 
Cataract,  hereditary,  199,  295 
CATTELL.    birthrate    of    college 

graduates,   431,   496 
families  of  scientists,  435 
Cause    and    effect,    universality 

of,   446,   447 

Causes,  natural  vs.  final,  164 
Celibacy,  409,  431 
Cell  characters  inherited,  199 
Cell   division,  81,  24,    110,   112, 

113,  114,  116,  120,  122 
differential,    119,    122,    183- 

186 
non-differential,     122,     183- 

186 
significance     of,     122,     123, 

125,  126 

Cell-Lineage,  diagram  of,  94 
CELLULAR    BASIS    OF    HEREDITY, 

89 

Centrifuged  eggs,  321,  322 
Centrosomes,   109 

equal  division  of,  123,  184 
Chances,  definition  of,  462,  463 
infinity  of  in  development, 

425,  426 
CHARACTERS,       developed       not 

transmitted,  351 
individual,  196 
inheritance      of      acquired, 

334-351 

inherited,  definition  of,  336, 
337 


latent,  209 

new  in  evolution,  388,  394, 
395 

not  independent,  193 

patent,  209 

racial,  195 
CHILD,    on    chromosomes,     163, 

164 

Choice  of  alternatives,  466 
j  Chorea,  295 
Chromatin,  24,  109 

granules,    110 
Chromomeres,  equal  division  of, 

184 
Chromosomes,  24,  111,   113 

abnormal   distribution,  311, 
313,  391,  392 

accessory,   140 

bivalent,  133 

conjugation  of,  131 

daughter,   113 

diploid,    137 

distribution,  115 

division,  111,  112,  123,  125, 
184 

haploid,   137 

identity,  113 

individuality,  115 

maternal  and  paternal,  117 

number   of,   111,   145-147 

"odd,"   140 

reduction  of,  136 

seat  of  factors,  164-167 

shuffle  and  deal  of,  158-160 

tetrads,  135 

X  and  Y,  143,   166 
Civilization,     means     good     en- 
vironment,  304 

vs.  heredity,  362 

will  it  endure,  396,  486,  401 
Classes,   hereditary,   413,'  414 

exclusive,   415 
CLEAVAGE,  of  egg,  24,  25,  26 

AND     DIFFERENTIATION,      110- 
126 

differential,  122 
non-differential,  122 
significance  of,  122,  123,  126 


518 


INDEX 


Cleavage     cells,     differentiation 

of,  26 
isolated,     development     of, ' 

314-322 

CLEPSINE,  behavior   of,  68 
Coeducation,  429,  430 
Cold,  induction  effect  on  Daph- 

nia,  349 
induction    effect    on    mice, 

349,  350 

Coloboma,  200,  296 
Color,  of  skin,  hair,   eyes,   197, 

291,  889',  292 
Color-blindness,  275-278 
CONSCIOUSNESS,  73-76 
continuity  of,  74 
loss  of,  75 
subconscious,  73 
CONTROL  OF,  alternatives,  466 
heredity    and    development, 
5,  367 

HUMAN   EVOLUTION,  403 

meaning  of,  461 

nature,  6 

phenomena,  367,  461 

self,  461 
CORRELATIONS      OF      GERM      AND 

SOMA,   162-179 
"Correlative        differentiation," 

331,  332 
CORRENS,  493,  497 

rediscovery     of     "Mendel's 
Law,"   224 

on  Mirabilis,  231,  232,  234, 

249,  265 
Creationism,  41 
Creative  Synthesis,  37,  84,  180 
CREPIDULA,  maturation  and  fer- 
tilization, 106,   107 

individuality  of  germ  nuc- 
lei, 116 

exogastrula  of,  324 
Cretin,  294,  331 
Criminality,  295,  445 
CTENOPHORE,  egg,  178 
CULTIVATED  PLANTS,  367,  368 

number  of  species,  368 


Culture,  grades  of,  396 

CUVIER,  473 

Cyclopia,  325 

CYTOPLASMIC  CORRELATIONS,   167 

differentiations,   180 

inheritance,   175-177 

localization,  123 

movements,  117 

Daltonism,  sex-linked,  275,  216, 

277,  278 

DAPHNIA,  effect  of  cold  on,  349 
DARBYSHIRE,  493 
DARWIN,  hypothesis  of  pangen- 

esis,   92 
on    domestic    pigeons,    368, 

369,  370 
inheritance       of       acquired 

characters,    335 
prepotency,  223 
reversion,  222,  223 
"sports,"  210 
.zeal,   472 

organism  a  microsome,  187 
theory    of    Natural    Selec- 
tion, 377,  494 
Daughter  nuclei,  24,  113 
DAVENPORT,  degrees  of  relation- 
ship, 217 

extra  toe  in  fowls,  267 
inheritance    of    skin    color, 

283-285 
Mendelian     inheritance     in 

man,  290 

transplanted  ovaries,  342 
"weakness    with    strength," 

426 

white  X  black  leghorns,  267 
494,  497 

DAVENPORT     and     WEEKS,     epi- 
lepsy inherited,  206 
Deaf-mutism,  200,  296 
Death,   of   families,   434-439 
Deathrate,  declining,  432-435 
Declaration     of     Independence, 
303 


INDEX 


519 


Decline  of  families  and  nations, 

487 

Defectives,  growing  burden  of, 
411 

alarming  increase  of,  420 
Defects,    educational    influence  I 

of,  356 
Democracy  and  human  equality, 

304 

DESCARTES,  303 
"Determinants"    of    Weismann, 

100,  255 
Determiners,  101 

combinations  of,  255 

differential  causes,  254 
DETERMINISM, 

and  RESPONSIBILITY,  443 

definition  of,  460 

not  FATALISM,  460 

not    predeterminism,   460 

of   ENVIRONMENT,   453-459 

of  HEREDITY,  44T-453 

of  personality,  459 

scientific,  460 

Development,    a    series    of    re- 
sponses, 327,  457 

alternatives  in,  462 

definition  of,  103 

is   transformation   not   new 
formation,  161 

mosaic,  333 

not  reversible,  462 

of  function,  36 

of  personality,  7,  77,  460 

potentialities  "of,  454,  455 

physiology  of,  307 

various  aspects  of,  76,  77 

viviparous,    18,    19 
DEVELOPMENT  OF  BODY,  8 

or  MIND,  39-41 

DEVELOPMENTAL  RESPONSES,  310- 
327 

AFTER  FERTILIZATION,  314 
BEFORE  FERTILIZATION,  310 
DURING  FERTILIZATION,  313 

DE    VRIES,    action    of   selection, 

377,  380 
fluctuations,  211,  212 


induction    effects    of    poor 
soil,  349 

intra-cellular       pangenesis, 
182 

"mutation  theory,"  211 

mutations,  211,  212 

Oenothera      mutants,     393, 
348 

on  nuclear  control  of  differ- 
entiation, 182 

pangenes,  100 

rediscovery     of     "Mendel's 

Law,"  224,  494 
Diabetes,  293 

Differential    division,    of    Cyto- 
plasm, 126 

of  cells,   119,   122,   183-187 
Differentiation,   37 

"correlative,"  331,  332 

definition  of,  103 

due   to   interaction    of   cell 
parts,   163,   180,   181,   182 

measure  of,  186 

nuclear  control  of,  181,  182 

"self,"  332,  333 
Dihybrid,  239,  241,  242,  244 
Dimples,  inheritance  of,  197 
DIONAEA,  reactions  of,  56,  58 
Diploid     number     of     chromo- 
somes, 137 

Disease,     inheritance     of,     200, 
202 

slight  resistance  to,  204 
Dislocation  of  organs  in  centri- 

fuged  eggs,  321,  322 
Dispermic  eggs,  313 
Divines,  poor  health,  357 
Division  period,  128 
Dogs,  different  races,  368 

psychological  characters  in- 
herited, 204 
DOMESTIC  ANIMALS,  367-374 

degree  of  change,  368 

how    produced,    375-377 

number  of  species,  367 

progenitors,  367,  368 

regressive  mutants,  394 


520 


INDEX 


DOMINANCE,    MODIFICATIONS    OF, 

265 

Blue  Andalusian,  266 
echinoderm  hybrids,  268 
extra  toe  in  fowls,  26T 
in   red   x   white   Mirabilis, 

265 

nature  of,  278 
not  fundamental,  279 
plain  x  banded  snails,  266 
red  X  white  cattle,  266 
sex-limited    characters,   270 
sex-linked   characters,    270- 

278 
white  X  black  leghorns,  267 

Dominant  characters,  227 
"extracted,"  230 
ratio  to  recessive,  233 

DONCASTER,  494 

Double  monsters,  315,  316,  322,  | 
328 

DRIESCH,  333,  494 

DROSOPHILA,  rapid  breeding  of, 

290 

sex-linked    characters,    166, 
271,  272,  273,  275 

DRYDEN,  206 

Duplex  Characters,  249 

Duty,  443,  459 
of  science,  480 

Dwarfs,   true,   293,  294 
caused  by  alcohol,  312 

Dynamic  equilibrium,  10 

Ear,  development,  28 

EAST,    heterozygosis,    384,    385, 
386,  497 

ECHINODERM  type  of  egg,  178 

Ectoderm,  25,  27,  28 

Education,  and  heredity,  428      i 
definition,  354 
good  and  bad,  354,  356,  357 
habit  formation,  457 
limiting  activities,  356,  357 
more  potent  in  man,  354 
needs  of  475 
possible  improvements,  489 


Egg  and  sperm,  hereditary  in- 
equality of,  176 

Egg  nucleus,  16,  24 

Egg     organization,     types     of, 
178 

ELLIS,  494 

ELSBERG,  plastidules,   100 

Emboitment,  81 

EMBRYOGENY,  26 

Embryology,   experimental,  307 

Embryonic  differentiation,  pro- 
cesses in,  179 

Embryos,  double,  315,  316,  322, 

323 

dwarf,  315 

half  and  three  quarter,  317, 
319 

Endoderm,  25,  27,  28 

"Energies  of  Men,"  472 

ENGELMANN,  49 

English  sparrow  in  U.  S.  432 

Engrammes,  348 

Environment,    acting    at    sensi- 
tive period,  375 
definition,  307 
direct  action  on  germ  cells, 

351,  348 

and   education,   360 
good  and  bad,  355,  356,  357, 

475,  476 
influence  in  producing  new 

races,  375,  376 
influence  on  ontogeny,  303, 

305 
influence  on  phylogeny,  302, 

305 

possible  improvements,  489 
social  institutions  and,  304 

Epidermolysis,   293 

Epigenesis,  81,  83,  161,  180,  452 

Epilepsy,  205,  206,  295 

Equality  of  Man,  303,  448 

ETHICAL   OBLIGATION,   484 

Ethics,  491 

Eugenicist,  methods  of,  416 

Eugenical   rules   as   to   defects, 
recessive,  421,  422 


INDEX 


521 


serious,  426-428 

slight,  426 
EUGENICS,  410-440 

contributory,    428-431 

declining  birthrate,  431-440 

definition  of,  412 

ideals,  412-419 

negative,   419-423 

only  hope,  449 

positive,  423-428 

Problems  in,  495 
EUTHENICS,  352 
Evolution,  control  of,  403,  404 

experimental,   305,  395 

progressive,   400 

promotion  of,  403,  488,  491 

requires      new     characters, 
388,  394,  395 

retrogressive,    400 
EVOLUTION  OF  MAN,  396-403 

contemporary,  398 

control  of,  403,  404 

future,  399,  400,  412 

intelligence  in,  403, 

natural  selection  in,  402 

prehistoric,  398 
Exogastrula,  824 
Experience,   factor  in  behavior, 
464 

learning  by,  65,  465 
Experimental  medicine,  5 
EXPERIMENTAL    STUDY    OF    IN- 
HERITANCE, 222,  224 
Eyes,  development,  28 

color,  197,  291 

lacking,  325 

fused  together,  325 


Facial  features,  inheritance  of, 

197 

German   type,  291 
Hapsburg  type,  291 
Jewish  type,  291 
FACTORS  OF  DEVELOPMENT,  79-86 
Factors,    added    in    progressive 
mutations,  394 


chemical   comparisons,   394, 

395 

definition  of,  101 
differential,  264 
distribution    in    maturation 

and  fertilization,  262 
dominant  and  recessive,  not 

modified  by  union,  343 
drop  out  in  regressive  mu- 
tations, 394 

extrinsic  and  intrinsic,  85 
for  color  developers,  256 
of  rabbits  and  mice,  259, 
of   sweet   peas,   256,   257, 

258 

for  pigment,  256 
location    in    cell,    261,    262, 

263,  264 

Mendelian,  264,  265 
multiple,  281-287 
nature  of,  260 
no  formation  de  novo,  395 
origin  of  new,  394,  395 
relations  to  characters,  255, 

256 

sex  determining,  265 
FAHLENBECK,  noble  families  of 

Sweden,  436 
FARADAY,  474 

Fat  stains,  effects  on  next  gen- 
eration, 350 

"Fate  of  part   function  of  po- 
sition," 333 
of  organization,  334 
Fecundity,  inherited,  204 
Feeble-mindedness,  205,  206, 295 
Feminist  movement,  484 
Fertility,  of   lower   types,  411 
FERTILIZATION,  15,  16,  107-110 

heterogeneous,  313 
"Fewer    and    better    children," 

439 
FISCHER,   mutations   of  insects, 

348,  497 

Fluctuations,  211,  212,  389 
Food,  influence  on  development 
in  tadpoles,   canaries,  bees, 
326,  327 


522 


INDEX 


FOOT  and  STROBE  LL,  on  chromo- 
somes,   163,    164 

on    sex-limited    characters, 

274 
FOREL,  494 

effect    of   alcohol   on    germ 

cells,  313 
FORMATION    OF    SUBSTANCES    IN 

CELLS,  180 
FORMULAE,     INHERITANCE,     244, 

246,  247 
Fowls,  races  of,  371,  372,  373 

transplanted    ovaries,    341, 

342 
FREEDOM,  and  determinism,  460  i 

birth  and  growth  of,  464 

definition   of,  466 

development  of,  465 

from  reproduction,  485 

greatest  in  man,  465     ' 

not  absolute,  459 

not  uncaused  activity,  466 

of  action,  72 

of  individual,  414,  443,  483 

of  society,  482 
Friedrich's  Disease,  295 
Frog,  behavior  of,  465 

double    embryos,    315,    316 
Function   and   structure,  corre- 
lated, 202 
FUNCTIONAL   ACTIVITY,  327 

in  human  development,  353 

GAGER,    radium   on   nuclear   di- 
vision, 311 
GALTON, 

age  of  marriage,  420,  431 

"Ancestral         Inheritance," 
215,  216,  223 

artistic  faculty,  192,  215 

characters,  192 

definition  of  eugenics,  412 

diseases,  192,  215 

eugenical   policy,  419,  420 

eye-color,   192,  215 

"Filial     Regression,"     218- 
220,  223 


genius  inherited,  206,  215 
heredity  vs.  civilization,  361 
intermarriage    of    scholars, 

415 

kinds  of  inheritance,  208 
nature  and  nurture,  301 
on  Ancient  Greeks,  405,  407 
on  identical  twins,  358 
pioneer  in  heredity,  193 
poor  health  of  divines,  357 
religious      significance      of 

evolution,  491 
on  "sports,"  210,  211 
statistical   study   of  inheri- 
tance, 214-223 
stature,   192,   196,  215 
weight  of  seeds,  192 

Gamete,  13 

Gardener,  methods  of,  416 

Gastrula,  25,  27 

GATES,  Oenothera  chromosomes, 
392 

GAUSS,  473 

"Gemmules,"     of     Darwin,     92, 
100 

Generalized   types,  413,  415 

Generations,    parental    and    fil- 
ial, 228,  230 
symbols  of,  231 

Genes,  101 

Genius,    and    physical    defects, 

357 

hereditary,  206 
unstable   nervous   organiza- 
tion, 206 

Genotype,  96,  241,  243,  245,  248, 
337,  381 

Geographical  isolation,  416 

GERMAN    EMPEROR,    number    of 
ancestors,  217 

GERM  CELLS,  8,  104 
alive,  9 

complexity   of   187 
possibilities   determined   in, 

448 
potential    personalities    in, 

438 
reactions  of,  464 


INDEX 


Germ  nuclei,  24 

individuality  of,  115,  116 
Germ  plasm,  in  nucleus,  123 

Theory    of    Weismann,    96,  j 

9T,  335,  336 
Germ  vs.  Soma,  96 
"Germ  track,"  diagram  of,  94 
GERMINAL  CONTINUITY,  92-97 
Glandular  secretions,  effects  of,  i 

329-331 

Glaucoma,  296 

GODDAHD,    feeble-mindedness   in-  j 
herited,  206,  494 

GOLDSCHMIDT,   494 

Gonia,  128 

Grafts,  not   modified  by   stock 

341,  342 

Great  men,  in  crises,  4T4 
GREECE,  decay  of,  485,  486 
GREEKS,   ancient,  405-408 
Growth  period,  128 
GUDERNATSCH,    effects    of    food 

on  tadpoles,  326,  497 
Guinea-pigs,   recombinations   of 
characters,  381,  382,  383 

transplanted    ovaries,    342- 1 

344,  346 
GUTHRIE,   transplanted   ovaries, 

341,  497 
GUYER,      on      chromosomes      of 

man,   147,  497 


Habits,    definition,    354 

good  and  bad,  354-356,  464 

HACKER,   494 

HAECKEL,  plastidules,   100 

HAEMOPHILIA,  203,  275,  296 

Hair,   color,   291 
form,  288,  291 

Half  castes,  of  Australia,  417 
of  New  Zealand,  417 

HANSEN,    mutations    of    yeast, 
348,  391 

Haploid     number     of     chromo- 
somes,  137 

Hardship,  educational  value  of, 
355,  475 


HARRIS,     induction     effects     of 

poor  soil,  349 
HARRISON,       on       transplanted 

limbs,  333 
graft  of  tadpoles,  341,  342, 

497 

HARSHBERGER,  498 
HARVEY,  epigram,  9 

epigenesis,  81 

HATSCHEK,  cleavage  and  gas- 
trulation  of  AMPHIOXITS, 
25 

larvae  of  AMPHIOXTJS,  27,  29 
Hereditary     lines,     interwoven, 

488 

HEREDITARY  RESEMBLANCES  AND 
DIFFERENCES,  193,  194,  207- 
214 

Heredity,   and  memory,  347 
and  variation,  194,  212 
control  of,  387 
definition  of,  89,  90,  103 
includes    assimilation,    etc., 

163 

mechanism  of,   105  ' 
more  potent  than  environ- 
ment, 438 

•  possible  improvements,  490 
theories  of,  105 
usually    unchanged    by    en- 
vironment,  437-447 
HEREDITY     AND     DEVELOPMENT, 

102-104 
HEREDITY    AND     ENVIRONMENT, 

85,  86,  302-306 
HEREDITY,    ENVIRONMENT, 

TRAINING,   357-359 
HEHING,    Organic    memory,    59, 

498 

Heritage,  definition  of,  103 
HERTWIG,  O.,  discovery  of  ferti- 
lization, 107 
human  ovum,  11 
idioblasts,  100  • 
influence  on  germ   cells   of 
X-rays,     radium,     chemi- 
cals/drug habit,  311,  313, 
498 

HERTWIG,  R.,  modification  of 
sex  ratio,  148-150,  498 


524 


INDEX 


Heterozygosis,  384,  385,  386,  387 
Heterozygotes,    228,    230,    241, 

243,  245 

Hindu  Dwarfs,  294 
HIPPOCRATES,  91 
HOMO  SAPIENS,  396,  402,  412 
NEANDERTHALENSIS,    397 
Homozygotes,  228,  230,  241,  242, 

245 

"Homunculus,"  80 
HOPPE,    Effect    of    alcohol    on 

germ  cells,  312,  498 
Human    embryo,    development, 

33,  35 
HUMAN  EVOLUTION,  CONTROL  OF, 

403 

slow,  438 

Human  faculties,  definition,  354 
Human  Heredity,   no  improve- 
ment in,  405-410 
Human  oosperm,  early  develop- 
ment, 31 
ovum,  11,  14 
spermatozoa,  14 

Humidity,    influence    on    muta- 
tion, 310 

Huntington's   Chorea,  295 
HUXLEY,  Evolution  and  Ethics, 

361,  498 

Hybrid   races,  quality  of,   418 
Hybridization,  human,  416,  417, 

418 

Hybrids,  increased  vigor,  384 
Hypertrophied    heart,    not    in- 
herited,  338,   340 
Hypophysis,      effects      on      de- 
velopment, 331 
Hypotrichosis,  293 
Hysteria,  206   295 

Ideals,    individual     and    social, 

413,  414 

Identity,  sense  of,  75 
"Idioblasts"  of  Hertwig,  100 
Idioplasm,  of  Nageli,  97 
Immigration,  407,  418,  419 

laws,   421 

Impulses,  conflicting,  465 
Inbreeding,   414,   415,   416 


INDIVIDUAL,  AND  RACE,  482 

minor  unit,  488,  489 
INDIVIDUAL    CHARACTERS,    196 
Morphological,  196 
Physiological,  202 

Psychological,  204 

Teratological,   199 
Individuals    and    their    charac- 
ters, 191 

"Induction,"    effect    of    colored 
soil,  350 

poor,  soil,  cold,  alcohol,  349 

not  inherited,  351 
Inequality  of  all  men,  448 
Infancy,  prolonged  in  man,  353, 

456 

Infertility,  causes  of,  436 
Inheritance,  "alternative,"  SOS 

"blending,"*  208 

of  baldness,  203 

cell  characters,   199 

dimples,    197 

facial  features,  197 

fecundity,  204 

genius,  206 

instincts,  204 

intellectual  capacity,  205 

left-handedness,  204 

longevity,  202,  203 

moral  tendency,  205 

obesity,   203 

"particulate,"   208 

pathological          characters, 
199-202 

physiological  characters,  202 

psychological        characters, 
204-207 

sex-limited   and   sex-linked, 
209 

stature,   196,   218,   219,   220 

temperament,   205 

teratological  characters,  199 

through  cytoplasm,  175-177 

tuberculosis,   201,   202 

will,  205 

INHERITANCE  FACTORS,  252 
Inheritance   material,   97 

seat  of,  162 


INDEX 


525 


Inheritance  units,  99,  102 
Inhibition,  68,  464 
Insanity,  205,  206,  295 
INSTINCTS,  52-56,  78 

altruistic,   467 

inherited,   204 

origin  of,  56 

reproductive,  484 
INTELLECT,  59-66 
Intellectual  capacity,  inherited, 
205 

genius,  291 

mediocrity,  291 

Intelligence,    factor   in    control, 
461 

in  evolution  of  man,  403      | 
Interaction  of  parts,  328-334 
Intra-cellular  pangenesis,  182 
INVERSE  SYMMETRY,  170-174 
Irritability,    12,   36 
ISOLATION     OF     SUBSTANCES     IN  I 
CELLS,  183,  185 
in  protozoa,  185 

JAMES,   WILLIAM,   472,   498 
JENNINGS,    action    of    selection, 

378 
behavior     of     Paramecium, 

62 

behavior  of  Stentor,  68 
inheritance  of  size  in  Para- 
mecium,  198 
on  Galton's  laws,  221 
on    potential    personalities, 

474 

rapid    breeding    of    Para- 
mecium,  290 

training  of  star-fish,  69,  498 
JEROME,  St.,  41 

Jews,  mixture  with  Gentiles,  417 
JOHANNSEN,  action  of  selection, 

377,  378 

genotype  and  phenotype,  96 
inherited    weight    of   seeds, 

197 

"pure  lines,"  378,  494 
JOHNSON,  marriages   of  college 

women,  430,  431,  498 
JORDAN,  D.  S.,  498 


JORDAN,  H.  E.,  498 

KAMMERER,  effects  of  colored 
soil  on  salamanders,  350, 
498 

KEIBEL,  development  of  human 
embryo,  31,  3S,  35 

KELLICOTT,  494 

Keratosis,  293 

KING,  modification  of  sex  ratio, 
149,  150,  499 

KORSCHELT  and  HEIDER,  Sym- 
metry of  egg  of  Musca,  169 

LAMARCK,  on  inheritance  of  ac- 
quired characters,  334 
LAMARCKIAN    HYPOTHESIS,    347, 

348 

Lamarckism,  38 
LANG,    snail   hybrids,   266,   267, 

499 

Laws  on  Eugenics,  420,  421 
Learning  by  experience,  65,  465 
Left-handedness,  inheritance  of, 

204 

Lens,  cataract,  199,  296 
development  of,  332 
displaced,  296 
weight  of,  199 

LEPTINOTARSA,  selection  in  379 
Life,    artificial    production    of, 

302,  304 

conditions   limited,  453 
definition  of,  9 
maze  of,  463 
LILLIE,    fertilization   of   NEREIS, 

17,  SO,  21 

on  fertilizin,  154,  499 
Limbs,   transplanted,   332,  333 
LINCOLN,  424 
LOCALIZATION  PATTERN,  172 

in  eggs  o-f  ctenophore,  flat- 
worm,  echinoderm,  anne- 
lid-mollusk,  chordate,  172, 
178 
Localization    of    substances    in 

cells,   185 
LOCK,  494 

LOEB,  J.  Artificial  partheno- 
genesis, 108,  152,  495 


526 


INDEX 


reflexes,  55  i 

tropisms,  54 

Logic,  as  test  of  truth,  451 
LOLIGO,  symmetry  of  egg,  169      ! 
Longevity  inherited,  202,  203 
Luxury,     cause     of     infertility, 
436,  437 

in  education,  475,  476  , 

MACFAHLAXE,    on    Dionaea,    56,1 
499 

Me   CLUXG,   on   sex   determina- 
tion, 141,   166,  499 

MAC      DOUGALL,      influence      of 
chemicals  on  ovules,  311,  499 

MAC    DOWELL,    size    in    rabbits, 
285-287,  499 

Me      GREGOR,      Restoration      of 
Pithecanthropus    skull,   397 

Male  babies,  greater  mortality, 
150 

MALTHUS,  theory  of,  420 

Man,  controls  destiny,  399 
dominant    races   of,   401 
evolution  of,  396-403 
extermination  of,  401,  402 
extinct  types  of,  396,  897 
freer  than   animals,  465 
mongrel   race,  425 
place  in  nature,  3 
prehistoric,   398 
races    of,   400 
species  of,  396,  397 
value   of   races   of,   400 

MAORIS  of  New  Zealand,  402 

Marriage,  age  of,  420,  431 
selection,  426-428 

Marsupials,  32 

MASSART,  reactions  of  SPIRILLA, 
51 

Materialism,  44 

"Maternal  impressions,"  34 

Matter  and  mind,  44 

MATURATION  PERIOD,  135 
divisions,    135,    137 

Maze,  of  heredity,  214,  297 
life,  463 

MECHAXISM    OF    DEVELOPMENT, 
179 


MECHANISM  OF  HEREDITY,  151 
Mechanistic   hypothesis,   479 
Mediocrity,  tendency  to,  218 
MEISCHER,  On  stereoisomeres  of 

albumin,  155 
MEMORY,  56-59,  78 
MENDEL,  abbot  of  Brlinn,  224 
dominant       and       recessive 

characters,  227-228 
dominant:    recessive    ratios, 

228-245 
experiments    on    peas,    193, 

224,  226,  499 
inheritance    formulae,    246, 

247 

inheritance  units,  255    • 
method   of   work,  225 
neglect  of  discoveries,  225 
purity   of   germ   cells,   234, 

235 
MEXDELIAN      ASSOCIATION      AND 

DISSOCIATION,  386-388 
Mendelian     factors    and    chro- 
mosomes,    165,     166,     262, 
263,  264,  265 
MENDELIAX      INHERITANCE      IN 

MAX,  288-296 
Table  of,  291-296 
MEXDELIAN  PRINCIPLES,  250 
DOMINANCE,  251 
MODIFICATIONS   AND   EXTEN- 
SIONS,  252 
SEGREGATION,  251 

UxiT    CHARACTERS,    250 

Mendelian    ratios,    simple,    228, 

230,  231,  232-V,m 
"back    cross,"    237,    238 
monohybrid,    340,    244 
dihybrid,  239,  $41,  242,  244 
trihybrid,  243,  244,  245 
dominant-recessive,    233 
departures   from,   280,   281, 
282 

MEXDELISM,   224-251,  297 

MEXDELSSOHN,       reactions       of 
Paramecium,   53 

Meniere's   disease,   295 

Mentality,    influence    of   educa- 
tion,' 303 


INDEX 


527 


Mesoderm,  27,  28 
Metabolism,  12,  36 
METCHINIKOFF,  disharmonies  in 

man,  361,  495 
Metempsychosis,  40 
Microscopic    particles,    smallest 

visible,  99 
Mind  and  body,  45 
Mind,   development   of,   42,  43 
MIHABILIS,  white-red  cross,  231, 

232,  234,  249,  265 
Mitosis,    21,   24,    110,   112,    113, 
114,  115,  116,  120 

significance     of,     122,     123,! 

125,  126 

"Mneme"  theory,  347,  348 
Modifiability  of  behavior,  68 
Molecular     constitution,     stere- 

oisomeres,   155 

Molecules,  largest  known,  99 
Monasticism,  409,  430,  431 
Monohybrid,  228,  236,  240,  244 
Monotremes,  32 

Monstrous     development,     309, 
454,   455 

cause  of,  310 
MONTGOMERY,   on   Chromosomes 

of  man,   147,  499 
Moral  qualities  inherited,  205 
MORGAK,   mutations    of   insects, 
348,   391,  495,  499 

sex  chromosome,   166,   167' 

sex  determination  in  Phyl- 
loxera, 150 

.  sex-linked  inheritance,  271, 
212,  274,  275,  276,  277, 
278 

rapid    breeding    of    Droso- 

phila,  290 
Morphological  characters,  196 

tests,  39 

Mosaic  development,  333,  318 
Moth  and  flame,  464 
MOTT,    insanity    inherited,    206 

495 

Mouse,  maturation   and   fertili- 
zation, 114. 


Movements,     within     eggs     and 
cleavage     cells,    50,    106, 
110,  117,  183. 
effects  of  stopping,  328 
random,  55 

of   spermatozoa,   49,   50 
Mulattoes,  skin  color,  282-285 
in  Jamaica,  417 
in  U.  S.  417 
MULLER,  JOHANNES,  478 
MULSOW,   132-135,  499 
Multiple    factors,    in    oats    and 

wheat,    281 
skin   color,   282-285 
size,  285-287 
Multiple  sclerosis,  295 
MUSCA,  symmetry  of  egg,  169 
Muscular  atrophy,  295 
Mutation  Theory,  305 
MUTATIONS,  209-211 

AND      FLUCTUATIONS,      211, 

212,  389 
progressive   and   regressive, 

394 
origin  of,  310-314,  348,  388- 

392 

Mutilations,  not  inherited,  339 
Myopia,  200 
NAGELI,  idioplasm,  97 

non-inheritance     of     alpine 

habit,  345,  495 
Natural  selection,  377,  380,  402, 

438 

nullified,   408-411 
Nature,  definition  of,  446 
man  part  of,  447 
mechanistic    conception    of, 

446 

vs.  nurture,  301 
stability    of,   399 
voluntaristic  conception  of, 

444 

NECTUHUS,  behavior  of,  69 
Neo-Darwinism,  45 
Neo-Lamarckism,  45 
NEREIS,  spermatozoon  of,  17 
maturation     and     fertiliza- 
tion of,  20,  21 


528 


INDEX 


NETTLESHIP,     hereditary     cata- 
ract, 198,  199,  499 
Neural  plate,  groove,  tube,  28, 

Neuritis  optica,  sex-linked,  296 

Neuropathy,  295 

New    England    families    dying 

out,  435 
NEWTON,  474 

Night  blindness,  sex-linked,  296 
NILSSON-EHLE,  multiple  factors, 

281,  282,  499 
Notochord,  29 
NUCLEAR  CORRELATIONS,  162 
Nuclear  division,  indirect,  21,  24, 

110,  112,  113,  114,  115,  116, 

120,  122,  123,  125,  126 
Nuclear  inheritance  theory,  162- 

167 
Nucleus,  10,  109,  113 

and     cytoplasm     concerned 

in   heredity,  177,  179 
Nulliplex  character,  249 

Obesity  inherited,  203 
OBSERVATIONS    ON   INHERITANCE, 

191 

"Odd"  chromosome,  140 
OENOTHERA,  mutants,  392,  39S 

chromosomes  of,  392 
Oneness  of  life,  5,  47 
Ontogeny  and  Phylogeny,  7,  302 
Oocytes,  129 

of  rabbit,  130 
Oogenesis     of     Ancyracanthus, 

134 

Oogonia,  128 
Oosperm,  13,  16 

double  cell,  18 

individuality  of,  18,  19,  22 

infection  of,  201 
Organ-forming     substances,    84 

in  Styela,  Amphioxus,  frog, 

118,  119 

Organism  of  humanity,  489 
Organization,   10 
ORGANOGENY,  28 
ORIGIN  OF  SEX  CELLS,  126-138 

DIVISION  PERIOD,  128 


Primitive   sex  cells,   128 

Oogonia,  128 

Spermatogonia,   128 

GROWTH  PERIOD,  129 

Oocytes,  129 

Spermatocytes,   129 

MATURATION  PERIOD,  135 
Orthogenesis,  305 
OSBORN,     Cartwright     Lectures, 

398,  499 

Otosclerosis,  200,  296 
Ovaries,   transplanted,  343-3^0 
OVIPARITY,  30 

Oviparous  development,  18,  19 
Ovules,  13 

Oxychromatin,    differential    dis- 
tribution, 184 

in  cell  body,  181 

"Pangenes"  of  deVries,  100 
Pangenesis,    hypothesis    of,    92, 

335 
PARAMECIUM,  avoiding  reaction, 

eg 

behavior  of,  61 

reactions  to  heat  and  cold, 
53 

races   differing  in  size,  198 

races  of,  205 

rapid   breeding   of,   290 

selection  in,  378 

trial  and  error,   63 
Parthenogenesis,  108 
"Particulate"  inheritance,  208 
Partition    walls    between    cells, 

185 

PASTEUR,  474 

PATHOLOGICAL    CHARACTERS    IN- 
HERITED, 199-202 
PEARL,  action  of  selection,  379, 

500 

PEARL  AND  PAHSHLEY,  modifica- 
tion of  sex  ratio,  149 
PEARSON,   ancestral   inheritance, 
216 

albinism  in  European  fam- 
ily, 292 

albinism  in  Papuan  family, 


INDEX 


529 


inheritance  of  tuberculosis, 
201,  600 

statistics,   fault   of,   221 
PEARSON    and   NETTLESHIP,  289, 

292,    500 
Permutations  in  distribution  of 

chromosomes,    156 
Personality,  determined  by  her- 
edity, 449 

development  of,  7,  77,  460 

infinity  of  chances  in,  425, 
426 

not   predetermined,   453 

potential,   474 

prediction  impossible,  424 
PHENOMENA  OF  DEVELOPMENT,  6 
Phenotype,   241,   243,   245,   248, 
381 

vs.  genotype,  96 
PHYLLOXERA,     degeneration     of 

male-producing      spermato- 
zoa, 150 

Physiological      characters,      in- 
heritance of,  202 

division  of  labor,  37 

processes,  10,  12 

states,  68,  464 

tests,  39,  54 

units,  100 
Pigeons,   behavior   of,  70,   71 

numerous    races,    368,    369, 

370 

Pineal  gland,  163 
PITHECANTHROPUS  ERECTUS,  skull, 

397 

"Plasomes"   of   Wiesner,   100 
Plasticity  of  behavior,  71 
"Plastidules"    of    Elsberg    and 

Haeckel,  100 
Plastosomes,   equal   division   of, 

184 

PLATE,     ancestors     of     German 
Emperor,  217 

factors    for   coat   colors   of 
mice,  259 

Mendelian     inheritance     in 

man,  290-296,  495,  500 
PLATO,  on  transmigration,  40 


Polar  bodies,  23,  138 

Polarity,    167-168 

of  "Styela  egg,  118,  120,  121 

Pollen,   13 

Polydactylism,    199,    293 

Polyhybrid,  244 

Population,     normally     station- 
ary,  432-434 
of  Europe,  433 

Poultry,  selection  for  egg  pro- 
duction, 379 

PREFOH.MATION,    79-81,    161,    452 

PREFORMATION   AND    EPIGENESIS, 
93-85 

"Preinduction,"  in  Daphnia,  349 

Prepotency,  223 

PRESENCE     AND     ABSENCE     HY- 
POTHESIS, 248-250 

Primitive  sex  cells,   128 

Principles    of    good    breeding, 
violation  of,  408 

Propagation    of   worst,    409 

PROTENOR,     sex     differentiation 
in,  142 

Protoplasm,   9 

Psychical   Anlagen,  47 

Psychical     development,     table 
of,    78 

PSYCHOLOGICAL    CHARACTERS   IN- 
HERITED, 204-207 

PTJNNBTT,  225,  258,  495 

"Pure  lines,"  378 

Puritans    and    Cavaliers    disap- 
pearing, 434 

Purity  of  germ  cells,  234,  235, 
251 

PYTHAGORAS,  on  transmigration, 
4.0 

Race  amalgamation,  402 

extermination,  402 

improvement,  6,  487,  488 

preservation,  484 
Radium,  disintegration  of  atom, 
388,  395,  396 

influence    on    spermatozoa, 

311 


530 


INDEX 


RANA,  grafted  tadpoles  of,  341, 

S4i 
Reactions,  of  germ  cells,  43,  49 

machine-like,  465 
REASON,   59-66 
Reception  cone,  109,  110 
Recessive  characters,  228 

"extracted,"  230 

ratio  to  dominant,  233 
Reduction  of  chromosomes,  136 
REFLEXES,  52-56 
Regeneration,      in      eggs      and 

adults,  318,  321 
Reproduction,  12,  36 
Responses,  varied,  68 
Responsibility,  443,  445,  459 

definition  of,  467 

of  society,  469,  470,  482 

varied,  468,  469 
Retinal  degeneration,  295 
RETZIUS,     human     spermatozoa, 

11,  14 

Reversible      changes      not      in- 
herited, 350 
Reversion,  209,  223 
Rickets,  not  inherited,  338,  340 
Rigidity  of  behavior,  71 
RIGNANO,  "Centre  -  epigenesis" 

theory,  347,  495 
ROMANES,  369,  370,  495 

cattle,  374 

fowls,  371,  872 

swine,  373 

ROME,  decay  of,  485,  486 
ROSANOFF,     insanity     inherited, 

206,  500 

Rotifers,     induction     effect     of  I 
alcohol,  349 

Salamanders,  effects  of  colored 

soil  on,  350 
SALEEB\',  495 
Savagery,  363,  396,  411 
Scholars,  prize,  415 
SCHULTZE,  double  frog  embryos, 

316 
Science,  duty  of,  480 


Segregation,   apparent   lack   of, 

280-287 
fundamental  to  Mendelism, 

279 
of   Mendelian   factors,   234, 

235,  251 

of  substances  in  cells,  183 
SELECTIVE  BREEDING,  only  meth- 
od    of     improving     race, 
404,  405 

Spartan   method,   404 
Self-control,  471,  475,  477,  478 
"Self    differentiation,"   332,   333 
Self  discovery,  471,  475 
SEMON,    "Mneme"    theory,    347, 

348,  495 

Sensitive  periods,  310 
SENSITIVITY',  48 

differential,  48,  52 
general,  52 
of  germ  cells,  49 
Sex,  a  Mendelian  character,  238, 

268,  269 

influence  of  food  and  tem- 
perature, 303 

Sex   cells,    fundamentally   alike, 
13 

SEX    DETERMINATION,     138-150 

in  human  embryo,   139 

in  man,   145,  146,  147 

McCLUNG  on,  141 

WILSON  on,  141 

STEVENS  on,  141 

Sex  glands,  effects  on  develop- 
ment,   329-331 
SEX-LIMITED    INHERITANCE,    209y 

268,  270,  274 
SEX-LINKED    INHERITANCE,    209, 

270-278,  296 
Sex  ratio,  modification  of,   140, 

148-150 
Sexual   reproduction,    value    of, 

157,  416 

SHAW,  BERNARD,  437 
Sheep,  cleavage  of  egg,  23 
SHTJLL,   heterozygosis,  384 
500 


INDEX 


531 


Significance    of    cleavage,    122, 
123,   126 

of  mitosis,  122,   123,   126 
Simplex  character,  249 
Skin  color,  197,  289,  291,  292 

mulatto,  282-285 

influence  of  light  on,  303 
Slow  breeding  of  man,  290,  438 
SOBOTTA,  fertilization  of  mouse, 

114 
Social  inheritance  vs.  germinal, 

360-363 
Social    institutions,     deal     only 

with  environment,  360 
Society,    highest    grade    of    or- 
ganization, 482 

power  of,  410 

responsibility  of,  482 

supreme  duty  of,  487,  488, 

491 
Soil,    poor,   induction   effect   on 

plants,  349 

SOMATIC  DISCONTINUITY,  92,  97 
Somatoplasm,  in  cell  body,  125 
Somites,  27,  29 

SPARTA,  destruction  of  unfit,  404 
Special  senses,  origin  of,  52 
Specialized  types,  413,  415 
SPECIFICITY  OF  GERM  CELLS,  151- 

161 
SPENCER,      physiological     units, 

100,  495 

Sperm  centrosome,  16 
Sperm  nucleus,  16,  24 
Spermatocytes,  129 
Spermatogenesis  of  Ancyracan- 

thus,  182 

Spermatogonia,  128 
Spermatozoon,   15,  17 

formation  of,  138 
Spina  bifida,  325 
Spinal  cord,  development,  28 
Spindle,  mitotic,  110,  111 
Spireme,  111 
SPIRILLA,  reactions  to  chemicals, 

51 


i  "Sports,"  210 
STANDFUSS,  mutations  of  insects, 

348 
1  Star-fish,  isolated  cleavage  cells, 

314 
Statistical     methods,      strength 

and  weakness  of,  219,  221 
STATISTICAL    STUDY    OF    INHERI- 
TANCE, 214-223 
Stature,  inheritance  of,  196,  218, 

219,  220 
tendency  to  mediocrity,  218, 

220 

influence  of  food  on,  303 
STENTOR,  modifiable  behavior,  68 
Sterile  insects,  440 
Sterility,  409,  435,  439 
Sterilization,  419,  421,  422,  439 
STEVENS,  on   sex  determination, 

141,  143,   166,  500 
Stimuli,  chemical  and  physical, 

467 

conflicting,   68,   464 
definition,   308 
DEVELOPMENTAL,  307,  310 
chemical,  308 
non-specific,  309 
physical,  308 
external    and    internal,    67, 

457,  464,  465 
range  of,  468 

rational,  social  ethical,  467 
summation  of,  57 
STOCKAHD,  alcohol  on  spermato- 
zoa, 311,  SIS,  500 
experimental  cyclopia,   153, 

325 

Structures  and  functions,  recip- 
rocal relations,  38,  45 
STYELA,     anterior     half-embry- 
os, 319 

dislocated   organs,   322 
egg  substances,  117-119 
gastrulation  and  larva,  124 
half  and  three-quarter  em- 
bryos,  317- 


532 


INDEX 


maturation,  fertilization 
and  cleavage,  119,  120, 
121 

posterior  half-embryos,  320 
Summation   of   Stimuli,   57 
SUMNER,  effect  of  cold  on  mice, 

349,  500 

Superman,  412,  439 
SWEDEN,  extinct  noble   families, 

436 

Swine,   wild   and   domestic,  373 
SYMMETRY,  168-170 
Synapsis,  131 
Syndactylism,   199,  293 

Tadpoles,   fed   on  thyroid,  thy- 

mus,  adrenal,  326 
Talents,  unused,  471 

parable  of,  473 
Temperament,     inheritance     of, 

205,  291 

Temperature,  influence   on  mu- 
tation, 310 

influence    on    cell    division, 

311 
TENEBRIO,  sex  differentiation  in, 

143 
TENNENT,    modified    dominance 

in  echinoderm  hybrids,  267, 

500 

Teratological  characters  inheri- 
ted, 199 
TERTULLIAN,  41 
Tetrads,  135 

THOMPSON,  cross   of  yellow   X 
green   peas,  229 

diagram     of     Gallon's     1st 

Law,  216,  495 
Thomsen's  disease,  295 
THORNDIKE,    behavior    of    dogs, 

cats,  monkeys,  64,  495 
"Thoroughbreds,"    415 
Thymus  gland,  fed  to  tadpoles, 

326 

Thyroid,  effects  on  development, 
331 

gland,  fed  to  tadpoles,  326 


Totipotence,   of    cleavage    cells, 

316,  318,  332 
TOWER,  action  of  selection,  379 

mutations   in   Leptinotarsa, 

310,  311,  348,  391 
\  Traducianism,  41 
Training  of  animals,  69 
Transmission  hypothesis,  91,  92 
Treasury     of     Human     Inheri- 
tance, 294,  495 
Trial  and  Error,  63 
Trihybrid,  243,  244,  ^5/381 
Triplets,  hereditary,  204 
Trophic  correlations,  329 
TROPISMS,  52-56 
TSCHERMAK,       rediscovery       of 

"Mendel's  Law,"  224 
Tuberculosis,      inheritance      of, 

201,   202 

Turbellarian  type  of  egg,  178 
Twins,    fraternal,    324 

hereditary,  204 

identical,  213,  323,  358,  457 

Ultra-microscopic  units,  100 
Uniqueness  of  every  individual, 

155,  212,  213 

UNIT  CHARACTERS,  250,  252,  253 
UNITS  OF  LIVING  MATTER,  97-102 

ultra  microscopic,  100 

units    of    growth    and    di- 
vision, 98,  99 

units  of  heredity,  99-102 
Unity  of  organism,  46 
Universal   laws,  451 
Use  and  disuse,  effects  of,  329 

effects  not  inherited,  339 
Uterus,  attachment  of  oosperm 

to,  31 

Variability,  caused  by  environ- 
ment, 388 

Variations,  continuous,  210 
discontinuous,  210 
fluctuations,  211,  212 
meristic,  210 
mutations,  211,  212 
"sports,"  210 


INDEX 


533 


Varied  responses,  68 
Vegetative  pole  of  egg,  167,  168 

VlVIPARITY,  30 

Viviparous  development,  18,  19 

WALTER,  diagram  of  Galtonian 

inheritance,  208 
filial  regression,  220,  495 
Wars,  effects  of,  422,  423 

shake    off    social    heredity, 

363 

Wasserman  test,  154 
WATASE,    symmetry    of    egg    of 

Loligo,  169 
WEEKS,  206,  501 
Weidal  test,  154 
WEISMANN,    determinants    and 

biophores,  100,  255 
germ  plasm  theory,  96,  100, 

335 

hereditary      and      environ- 
mental variations,  212 
on    differential    division    of 

chromosomes,  125 
inheritance       of      acquired 

characters,  335 
nuclear    control    of    differ- , 

entiation,  181 
reduction   of   chromosomes, 

137,  496 
WHITMAN,  behavior  of  clepsine, 

68 

Necturus,  69 
pigeons,  70,  71 
freedom  and  choice,  71,  72, ! 

501 

WHITNEY,   induction   effects   ofi 
alcohol,  349,  501 


WIESNER,  plasomes,  100,  501 
WILDER,    duplicate    twins    afcd 

double  monsters,  323,  501 
WILL,  67,  72,  459 

absolutely  free,  445 

denned,  470 

good  and  evil,  445 

inherited,  205 

nature,  expression  of,  444 

supreme  faculty,  471 

training  of,  470,  471 
WILSON,  distribution  of  factors, 
262 

of  chromosomes,  263 

dwarf  and  double  Amphi- 
oxus  embryos,  315 

on   sex   determination,   141, 
142,  144,  166,  167,  496,  501 
WINIWARTEH,    on    chromosomes 
of  man,  145-^6 

oocytes  of  rabbit,  130,  501 
WOLFF,  "Theoria  Generationis," 

82,  501 
WOLTERECK,  induction  effects  of 

cold,   349 

preinduction,  349,  501 
Women's  Colleges,  influence  on 

marriage,   430,   431 
WOODS,  "Heredity  in  Royalty," 


X-rays,  influence  on  spermato- 
zoa,  311 

X  and  Y  chromosomes,  143,  166 
Yolk,  influence  on  size  of  egg, 
13,   14 

Zygote,  13 


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